update HotwordDetector exception throws

A few updates to throwing exceptions in AOHD:
- overrideAvailability throws exception when overriding to ENROLLED
  state without an enrolled model
- APIs no longer throw runtime exceptions when it could be due to
  system server modifying the detector state without the caller's
  knowledge. In cases where a RuntimeException was thrown, a checked
  exception is thrown instead.

Bug: 226355112
Test: atest AlwaysOnHotwordDetectorTest
Test: atest AlwaysOnHotwordDetectorSystemApiTest
Change-Id: Id6884a325a1cd79f1cc880f370a0a97e4ab0ec04