add full battery saver runtime modification API

New system API added for privledged clients to modify the full battery
saver mode policy. This API is used to override static settings at
runtime, and any overridden settings are cleared when exiting full
battery saver mode.

Bug: 172294448
Test: atest PowerManagerTest
Test: atest PowerManagerServiceTest
Test: atest BatterySaverPolicyTest
Test: build and boot ensuring SoundTrigger service behavior is not
changed

Change-Id: I41f968799b184a5dac702553379294795614be0a