Improving BatterySaverAlarmTest

Making some reliability improvements to reduce flakiness and latency of
the test.

- Changing simple sleeps to polling checks where appropriate
- Stopping the app more reliably
- Splitting tests into smaller focussed tests

Flag: TEST_ONLY

Test: CtsBatterySavingTestCases:BatterySaverAlarmTest

Bug: 399966290
Change-Id: I48a587105caaf97f96c14a4916613a0ed09503e9