Fade transitions between messages for voicemails.

- Change playback layout to have two superimposed text views, both
  visible, one transparent and the other opaque.
- When swapping between permanent and temporary text, fade out one view
  and fade in the other.
- Also uses the constrain() method from MathUtils and get rid of my
  clamp() method.

Bug: 5113788
Change-Id: Ia72a3f2a85fd9ac32303e7cf18744a12ba46f75f