Request enter PIP on WCT#setWindowingMode(PINNED)

Check that the task is able to enter PIP mode and let the app
decide whether they want to enter instead of forcing any
task into PIP mode when the windowing mode is set through a
WindowContainerTransaction.

Bug: 213891603
Test: atest WindowContainerTests; atest WindowOrganizerTests;
atest ActivityTaskManagerServiceTests; manually, set PINNED win
mode through a wct, verify the app is requested to enter PIP
before actually putting the task into PIP mode.

Change-Id: I703b172fdca01124e92cd3cbce7765166b5b4f01