Stop screen sharing when exiting MediaProjectionDemo.

The existing implementation will leave screen sharing running after
stop, causing the VirtualDisplay to not be destroyed. This change
makes sure any screen sharing ends on stop.

This change also addresses the the ToggleButton's state being saved.

Test: manual
Bug: 26221019
Change-Id: I51ac8bf09932a83100776c919146eed8c28f35b3
1 file changed