Add ActivityListener to VirtualDeviceManager

The API allows virtual device owners to listen to when the top activity
of a virtual device's display changes, and when a display becomes empty.
The former is used to display a notification to the user that can say
"Syncing <app name> to your Chromebook". The latter is used so that the
streamed window on ChromeOS side can be closed when the display becomes
empty.

Bug: 194949534
Test: To be added in ag/15781910
Change-Id: I5172e55a3a79b5fadc1719e99160b7675612e97b