Always play remote hold tone if carrier config indicates it should be.

Remote hold tones are normally only played when the audio stream goes
inactive and the call is remotely held.
Adding support for carriers which do not make the audio stream go inactive
when the call is remotely held.

Test: Added unit test coverage for hold tones, manually tested with TMO.
Bug: 64099717
Change-Id: Ib001cbc7613551302d87c1a9a5f5e6412f1cbc57