Suppress DTMF tone start when a call is already held.

It is possible that the Dialer UX can send a DTMF tone start when a call
is already held.  Detecting this and skipping the DTMF tone play in that
circumstance.

Test: Manually verified issue; added new unit tests.
Bug: 33771011
Change-Id: Ib063237d768fc2f6c512d81d356ad0874410ce7b