Deactivate touch target for splitscreen button

Remove ripple after touching the splitscreen button
(with id of android:id="@+id/enter_split" defined in pip_menu.xml)
by setting the ImageButton setEnabled to true in PipMenuView.java

Test: atest SystemUITests
Test: manually, reproduced steps in the bug listed
Bug: 239613989
Change-Id: I105966feb22a0ccc5f11b59848c81f9ba682c3eb