ProfileService: Change `start` return to void

All implementations of ProfileService#start always return true.

So the return is changed from boolean to void to be able to remove
all checks of ProfileService#start's return value and simplify
the callers.

Bug: 319112725
Test: m Bluetooth BluetoothInstrumentationTests
Flag: Exempt, mechanical refactor
Change-Id: Icffc4065bdd8ff680804ccb934cee0c22dcb3011