Check Activity state before PiP state update

Check Activity state before sending the PictureInPictureUiState update
to the client. Activity could have been destroyed in app process, while
it's still attached.

Flag: EXEMPT testfix
Bug: 412980276
Test: atest CtsWindowManagerDeviceActivity
Change-Id: I13bc246099e23c21c8e16bfb97f5259ffc2095d3