Fix cross-fade legacy enter PiP2

Here is the list of changes related to cross-fade:

1. Use the correct PiP leash from PipTransitionState
2. Set the correct initial position and crop
3. Add a duration for alpha animator internally
4. Make sure PipTransitionState is ENTERED_PIP afterwards
5. Allow TRANSIT_OPEN mode to trigger legacy enter too,
since that's what we use in multi-activity cases

This CL also addresses recent changes to enter-PiP with
icon overlay logic, where a custom sourceRectHint is used
if apps provide an invalid or null one.
This should simplify handleSwipePipToHomeTransition(),
which uses a new getEnterPipWithOverlaySrcRectHint() static method.

Bug: 322871963
Test: legacy-enter PiP
Test: auto-enter PiP with an invalid or null srcRectHint
Change-Id: I559b6be7fac72a833519e2d80a7d86a6d4b80788