Instrumentation test should not toggle flags

Since these test can run against optimized release device that doesn't
have the inactive code path corresponding to a flag, toggling the flag
in test end up in systemic failure

Bug: 400865832
Bug: 400865186
Test: atest BluetoothInstrumentationTests on release target device (MTS)
Flag: TEST_ONLY
Bug: 375466974
Flag: com.android.bluetooth.flags.opp_check_content_uri_permissions
Bug: 388159341
Flag: com.android.bluetooth.flags.non_conference_call_hangup
Bug: 345380335
Flag: com.android.bluetooth.flags.maintain_call_index_after_conference
Ignore-AOSP-First: most of the test are internal only
Change-Id: Ia787328f3e0a3d35dd456907965ccc6afde8e638