Add additional tests for classes in package hfp

This CL adds tests for:
- HeadsetAgIndicatorEnableState
- HeadsetService
- HeadsetStackEvent
- HeadsetStateMachine

Bug: 237467631
Test: Ran each test with atest
Change-Id: I07d290a65d77cbfe7e0d5203b0f81d511e3ece7f
Merged-In: I07d290a65d77cbfe7e0d5203b0f81d511e3ece7f
(cherry picked from commit 8d0ac9e0)