Need to start app immediately after the boot completed

This can increase you priority but still there would be some delay. Since android first load its OS and the all the other activity starts.

<receiver
    android:name=".AutoStart"
    android:enabled="true"
    android:exported="true"
    <intent-filter android:priority="1000">
        <action android:name="android.intent.action.BOOT_COMPLETED" />
    </intent-filter>
</receiver>