Implement content overlay in PiP2

Make sure the direct enter PiP and auto-enter PiP
with invalid source-rect-hint use app icon overlay
during the enter-animation.

Once the enter animation is done, we start fading the overlay out.

Note: with PiP2, we can finish the transition and set the state to
ENTERED_PIP right away even before overlay fadeout is done.
This allows PiP interactions earlier than in PiP1.

Bug: 373946868
Flag: com.android.wm.shell.enable_pip2
Test: auto-enter PiP in btn-nav with invalid src-rect-hint
Change-Id: I2156b7890243c13f278d6023802e41e69db09683