add SoundTrigger service modes in battery saver

Control over the SoundTrigger service behavior in battery saver mode is
expanded to from a boolean to multiple modes. Modes include enabled,
disabled, and privileged. Adding the privedged mode allows for the
SoundTrigger service to selectively control clients which are deemed
esential to the Android system.

Bug: 172294448
Test: atest BatterySaverPolicyTest
Test: atest CtsBatterySavingTestCases
Test: atest PowerManagerTest
Test: build and verify backward compatibility with SoundTrigger system
service behavior

Change-Id: Ib701963b07b205e5902ef265198b390a9850cb88