Guard FallbackHome activity behind permission

An attacker can launch FallbackHome of the Settings app
to reset unlock time. This would prevent the device from
sleeping because it runs the following code:
PowerManager#userActivity().

However, running PowerManager.userActivity() requires
android.permission.DEVICE_POWER or android.permission.USER_ACTIVITY.

So, let's add a permission to guard this activity.

Test: Rebuilt a rom and see no side effect.
Bug: 246543238
Change-Id: I23a1e352f76cbec3e8c07a1b58b94615f2779121