BtHelper/SoundDoseHelper: fix misc error-prone warnings

Delete unused constant.
Suppress permission warnings since calling from system-server.
Change GuardedBy annotations to comments when just documenting
  that a method is called from another class under a lock of
  that class.
Fix calls to Handler.sendMessageAtTime that were using a delay
  (always at 0) instead of uptime since boot.
Replace StringBuilder by String concatenation.

Bug: 341995844
Test: m -j RUN_ERROR_PRONE=true services
Flag: EXEMPT bugfix
Change-Id: Idde17e8bbf5fcc2585b4aeb11d49e9bd02f64a11