HearingAidService: Cache service in getService

If cleanup is callled at the same time than getService, mService
will become null and call to methods expecting the argument to not
be null will fail.

mService being null is checked by `Utils.checkServiceAvailable`,
to avoid the subsequent `Utils.check` calls failure, cache the
value to ensure that it will not change.

Bug: 327929337
Fix: 327929337
Test: m Bluetooth
Flag: Exempt, strict null check
Change-Id: I8a533e0c5df2bd7f39a493f5247e2198272682ba