Target system_current for PermissionController.

This is a partial cherry-pick of ag/16185351, which updated the
target_sdk_version and added the explicit android:exported attributes
and PendingIntent.FLAG_IMMUTABLE flags.

Fixes: 211369973
Test: presubmit
Merged-In: I8d145248221868f1e69453958543ed66c1648037
Merged-In: Ie76983423f81b6503d4f24f2d0ba62875593fd7c
Change-Id: Ie76983423f81b6503d4f24f2d0ba62875593fd7c