SF: acquire/release screen for active display only

SurfaceFlinger assumes that there is at most a single internal display
powered on at a given time, and mark it as the active display. However,
in order to be robust against rare race conditions where displays might
be on together for a short period of time, we add a check to make sure
that we tell the scheduler that the screen was acquired/released only
for the active display.

Bug: 201605862
Test: SF unit tests
Change-Id: I25b3f807d9f5d93ae88ac8a6026cee76cb69f493