countDown.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    tools:layout_editor_absoluteY="81dp">

    <SeekBar
        android:id="@+id/seekBar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        app:layout_constraintBottom_toTopOf="@+id/imageEgg"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <ImageView
        android:id="@+id/imageEgg"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="34dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:src="@drawable/egg" />

    <TextView
        android:id="@+id/tv_Countdown"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="18dp"
        android:layout_marginRight="18dp"
        android:layout_marginBottom="25dp"
        android:text="0:30"
        android:textSize="25sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/btnStartStop"
        app:layout_constraintEnd_toEndOf="@+id/btnStartStop" />

    <Button
        android:id="@+id/btnStartStop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="152dp"
        android:onClick="pressToStart"
        android:text="Start"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
    <Button
        android:id="@+id/btnStop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="90dp"
        android:text="Start"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />


</android.support.constraint.ConstraintLayout>