Polish enter PiP animation on back pressed

The PipTransitionAnimation#ofBounds is used when entering PiP on back pressed.
- Adjust the scale based on current animation progress
- Alter the auto-enter-pip animation running in Launcher to use the same
  mechanism
- Update the ofBounds animation to ignore small source rect hint,
  similar to what's in auto-enter-pip animation running in Launcher

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dv35ov6nK2Mrr98DGkLNaE
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/gm8Kl7N3NPIoWoDKyDfd5s
Bug: 235268241
Test: Verify enter PiP animation on back pressed with YouTube in \
      both folded and unfolded state. See also videos.
Change-Id: I6043cff31eed7b63087ea03b17ced7549c89e4fb