Make sure Activity is attached before IPC call

When trying to send the PictureInPictureUiStateChanged callback to
Activity, the app process could have been killed.

The Activity state check alone is not sufficient enough and an
additional attachedToProcess check is added in this change.

Flag: EXEMPT bugfix
Bug: 407221296
Test: atest PinnedStackTests
Change-Id: Ic1aaa9c1a54291be879413d0ee60c9d7e653080b