Deprecate pip-to-split function

This function never goes to production and we have concluded that
app should exit to full-screen mode when it leaves PiP.

Bug: 323420632
Flag: N/A
Test: atest PinnedStackTests
Test: m -j SystemUIGoogle ArcSystemUI
Change-Id: I49a9f0d57bf57673cbf1b0ad1ee45ffa96cfabfd