Add #onPictureInPictureRequested to Activity and ATM

Allows vendors to signal that an activity should enter
picture-in-picture if possible.
Additionally, removes the need for an Activity to go
through onPause to enter picture-in-picture. Apps can
now override this new API and enter PIP from there
instead of relying on #onUserLeaveHint.

Bug: 143365086
Test: atest FrameworksCoreTests:android.app.activity.ActivityThreadTest
Test: atest CtsWindowManagerDeviceTestCases:PinnedStackTests
Test: atest WmTests:ActivityTaskManagerServiceTests
Change-Id: Ib7ae9d1a7055ceed73e9643982033de9d4ad7350