HeadsetTest: cleanup invalid check and comments

Calling new in java cannot return null -> remove check
calling init is synchrone, we do not need to wait with a timeout
The comment about ASYNC_CALL_TIMEOUT_MILLIS is not correct since we are
waiting more than 250ms and we do not consider this to be a breakage.

Bug: 338303046
Test: atest HeadsetServiceAndStateMachineTest
Test: atest HeadsetServiceTest
Flag: Exempt, test only
Change-Id: Icba0e20057ed5357cd5e4dc3345535161e6585a1