Fix location-retention regression: stop GPS when backgrounded and idle

The previous fix (48ae3b0) dropped to ECONOMY (5 s interval) on app
pause, but never stopped location requests. The fusedLocationClient
continued polling indefinitely in the background even with nothing
recording or anchor-watching — accessing location without any need.

Fix:
- Add PowerMode.NONE to represent "no updates in flight"
- Add ACTION_STOP_UPDATES to LocationService: calls
  stopLocationUpdatesInternal() and parks _currentPowerMode at NONE
- Guard startLocationUpdatesInternal() against PowerMode.NONE so a
  stray setPowerMode(NONE) call can never request zero-interval updates
- Change _currentPowerMode initial value from FULL to NONE (accurate:
  no updates are running at process start)
- throttleLocationIfIdle() now sends ACTION_STOP_UPDATES instead of
  ACTION_START_ECONOMY; the existing restoreLocationMode() on onResume
  already sends ACTION_START_ECONOMY which transitions NONE→ECONOMY
  correctly since NONE != ECONOMY bypasses the no-op guard

Tracking and anchor-watch paths are unchanged: throttleLocationIfIdle()
skips the stop if either is active, and onResume sends ACTION_START_FULL
when recording is still in progress.

https://claude.ai/code/session_0126zvB69ccVoFtx2o96oaE2

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