waitForAllDevicesReady.gradle

import com.android.build.gradle.internal.tasks.DeviceProviderInstrumentTestTask
import com.android.builder.testing.api.DeviceConnector

def isDeviceReady(DeviceConnector device) {
    def res = device.getSystemProperty('init.svc.bootanim').get()
    return res == 'stopped'
}

def waitForAllDevicesReady(DeviceProviderInstrumentTestTask testTaks) {
    // Add time out?
    def devices = testTaks.deviceProvider.devices.clone()

    while (!devices.isEmpty()) {
        devices.removeAll isDeviceReady
        sleep(1000)
    }
}    

android.testVariants.all { variant ->
    variant.connectedInstrumentTest.doFirst {
        waitForAllDevicesReady(variant.connectedInstrumentTest)
    }
}