Refactor Voicemail Playback into standalone view.

+ Substitutes the existing playback widget in CallDetailActivity,
although the plan is to move this to the call log shortly.
+ Convert the widget from a fragment into a layout. This allows us
to more easily create multiple instances of the voicemail widget in
the same view, as we intend to do in the call log.
+ Shift UI-related logic from Presenter to the Layout.
+ Fix janky seeking, so that it now works correctly consistently
rather than sporadically, and doesn't need to buffer again.
- Remove the VariableSpeed player formerly used in the Presenter. We
don't use this functionality anymore, and this allows us to directly
used the framework MediaPlayer (instead of a custom legacy proxy).

Bug: 21170557
Bug: 20693172
Change-Id: Ia34f459df10e43763b32fdb0954f83e882664231