Merge sleeping activities with visibility updates

Previously we were using a separate SLEEP message to communicate
with the client. The sleeping implementation on the client though
simply transitioned it between STARTED and STOPPED states.

This CL relies on visibility updates to communicate the sleeping
state. The "sleeping" status field is still used in the server-side
to record when device started going to sleep, and prevents any
activity that was launched before to influence the power state of
the device.

Bug: 137329632
Test: AM/WM CTS and unit tests
Change-Id: Idd7f5bf6730d3372168d862ef713a44f700430f1