Check that PIP is active before setting the PIP exclusion bounds

Fixes issue that caused the exclusion bounds to be set to the
last PIP bounds even if the PIP window did not exist anymore,
leading to the back gesture to not work.

Bug: 176302696
Test: enter PIP, stash, exit via drag to bottom, enter dark mode -
the back gesture should work on the region that was previously stashed
pip

Change-Id: Ife9abf6382c7d973d9cefbc6a3dc1f355989fa42