[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

The test flaky on the previously reverted CL is passing (10 runs):
https://android-build.corp.google.com/abtd/run/L62000030012644815/

Note: the only difference from the reverted ag/33579244 is the
additional check in ActivityRecord#updatePictureInPictureMode(),
where we take into account that we could be launching into PiP.

Bug: 400459203
Bug: 420614827
Flag: com.android.wm.shell.enable_pip2
Test: atest ContentPipHandlerTest
Test: atest PipSchedulerTest
Test: atest ActivityStarterTests
Test: atest PinnedStackTests
Change-Id: If64a9a516e4de75ff20691a9aa76deae69b063cd