Add permissions to android project's manifest on build time

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="in.rahulkr.buildtimepermissions">
    <!--Below commented tags are necessary because we are using regex to find and replace permissions-->
    <!-- START:PERMISSIONS -->
        <uses-permission android:name="android.permission.BATTERY_STATS"/>
        <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>
    <!-- END:PERMISSIONS -->
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

build.gradle

import java.util.regex.Matcher;
import java.util.regex.Pattern;
apply plugin: 'com.android.application'

android {
    compileSdkVersion 25
    buildToolsVersion '25.0.2'
    defaultConfig {
        applicationId "in.rahulkr.buildtimepermissions"
        minSdkVersion 19
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
//    products {
//       android
//    }
    applicationVariants.all { variant ->
        variant.outputs.each { output ->
            output.processManifest.doLast {

                // 1. read the file permissions.json which contains a list of required permissions
                // 2. parse the permissions from json to create a string containing permissions
                //    as they should appear in manifest
                // 3. read the manifest file for each variant
                // 4. match manifest content against the regex
                // 5. replace the match with string from step 2
                // 6. write the new content to manifest file
                
                def inputFile = new File("app/permissions.json")
                def json = new groovy.json.JsonSlurper().parseText(inputFile.text)

                def permissions = "\n<!-- START:PERMISSIONS -->"

                json.permissions.each{permissions+="\n<uses-permission android:name=\""+it.name+"\"/>"}
                permissions+="\n<!-- END:PERMISSIONS -->"

                def manifestFile = output.processManifest.manifestOutputFile

                // If you get below error on build time, it's probably because your gradle version is newer than mine
                // Error: Manifest Tasks does not support the manifestOutputFile property any more, please use the manifestOutputDirectory instead.
                // comment above line which defines 'manifestFile' and uncomment below line
                
                // def manifestFile = file("$manifestOutputDirectory/AndroidManifest.xml")
                
                def content = manifestFile.getText()

                def pattern = Pattern.compile("<!--\\sSTART:PERMISSIONS\\s-->.*<!--\\sEND:PERMISSIONS\\s-->", Pattern.DOTALL);
                content = pattern.matcher(content).replaceAll(permissions)

                manifestFile.write(content)
            }

//        variant.processResources.manifestFile = file("${buildDir}/filtered_manifests/${variant.dirName}/AndroidManifest.xml")
        }
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:25.+'
    compile 'com.android.support.constraint:constraint-layout:1.0.1'
    testCompile 'junit:junit:4.12'
}

permissions.json

{
	"permissions": [
		{"name": "android.permission.WRITE_EXTERNAL_STORAGE", "optional":"true", "description":""},
		{"name": "android.permission.ACCESS_COARSE_LOCATION", "optional":"true", "description":""},
		{"name": "android.permission.READ_EXTERNAL_STORAGE", "optional":"true", "description":""},
		{"name": "android.permission.ACCESS_NETWORK_STATE", "optional":"true", "description":""},
		{"name": "android.permission.CAMERA", "optional":"true", "description":""},
		{"name": "android.permission.INTERNET", "optional":"true", "description":""},
		{"name": "android.permission.BLUETOOTH", "optional":"true", "description":""}
	]
}