WifiActivityEnergyInfo: clean up API surface

- Removed setters and made object immutable.
- Added annotations where appropriate.
- Marked isValid() as @hide.
- Refactored BatteryExternalStatsWorker to not
    use setters

Bug: 148218218
Test: compiles
Change-Id: I799149f5ec6a88192f686cc23e947466765b2a1e
3 files changed