Make enable_pip_ui_state_on_entering android.app

Move the aconfig enable_pip_ui_state_on_entering from
com.android.wm.shell to android.app package under the same multitasking
namespace.

Renamed also from enable_pip_ui_state_on_entering to
enable_pip_ui_state_callback_on_entering to avoid confusions in gantry.

The flag should be accessible from WM-server and WM-shell,
shown under the same "Android Platform: Multitasking" section in gantry
after this change.

Bug: 303718131
Flag: ENABLE_PIP_UI_STATE_CALLBACK_ON_ENTERING
Test: N/A
Change-Id: I639089ff591cb68d2a3b767e15ec2f043b5aaf32