Fix inaccuracies in BatterySaverPolicy cache invalidation

Subsequent changes to BatterySaverPolicy changed the variables
isBatterySaveMode depends on, causing us to fail to invalidate at the
correct time. This change moves the invalidate to occur alongside the
proper update to state.

Bug: 153712245
Test: built and flashed
Change-Id: Iab981286b321a6e9f235e01a62a3ea7cb13d4133