2019.4 dev

1. Integrate busybox_nh to the app and it will be used if user has no busybox_nh installed yet.
2. Re-implemented the bootkali_env, all other scripts should source it first to get the variables like $BUSYBOX, $MNT, etc...
3. Preserve the android env to the kali env.
4. Bind mount the android /system partition to kalichroot/system as well.
5. Moved disable battery optimization check to CompatCheckService class.
6. Updated Gradle and targetSdkVersion to 29