Introduce Background Activity Launch home mode

Background activity launch home mode will replace existing app
switch protection logic.

After user processing home button, all normal apps, including
ui visible apps, will no longer able to start background
activity even its within grace period, except the apps that
have permission to start background activity. This will stop
apps try to start itself during onPause() after pressing home button.

It removes the pending activity queue in the origianl
apps switching protection due to confusing UX. Also, it removes
the logic that stop app switching protection after 5 second timeout.

Bug: 159433730
Test: Create an app that startActivity in onPause(), then click home
button. Activity can be started in its background task but not a new
foreground task.
Test: Create an app and enter pip mode in background and start new
activity task. After pressing home button, apps can enter pip mode
but cannot create a new task.

Change-Id: Iaa77daba2fbce68f65a60ca71969c3f18bac3684