Redo DtmfLocalTonePlayer

The system of locks in DtmfLocalTonePlayer and the constant thread
creation/destruction seems to be behind a system ANR bug. This change
refactors the class to rely on a single Handler and associated
HandlerThread that lives for the entire duration of Telecom.

Bug: 34886553
Test: manual and unit
Change-Id: I1d86c644aaecdb6ff93d31e2b9e97792e3e301e0