Stopping target app in BatterySaverAlarmTest tearDown

If the helper target app is not reinstalled between subsequent tests in
the same module, this can leak state and cause unreliable failures.

Test: atest CtsBatterySavingTestCases

Flag: EXEMPT TEST_ONLY

Bug: 440159306
Change-Id: I6270144e44157241528c363e94647731d5134f50