Persist PiP size to be restored on re-entry

Adds support for restoring the PiP size along with its snap fraction.
Modifies PipBoundsHandler logic to use both the previous size and
position (snap fraction) when calculaing the bounds for re-entry.
Additionally, some phone-specific logic is added to ensure that the
re-entry size that is restored is the non-expanded size (i.e. the
size before the bounds are animated to expand when pip is clicked).

Bug: 143641277
Test: enter pip, move it, touch to expand, restore. Then re-enter
pip. Pip should retain its size and position.
Test: atest PinnedStackTests
Change-Id: I21b7e4bf9360cd7da9faf2015c04d12d37c0241f