[PiP2] Implement Content-PiP2

Allow entering content-pip in PiP2 via launching
an activity with launchIntoPip ActivityOptions.

Also wire exit-via-expand to just PiP fadeout removal
with an extra side-effect of starting the original host task

Bug: 400459203
Flag: com.android.wm.shell.enable_pip2
Test: atest ContentPipHandlerTest
Test: atest PipSchedulerTest
Test: atest ActivityStarterTests
Change-Id: I804dafae368e4be298ce430960b0d33e9078e727