Add the notion of a last callback time into DispSync.

If we do not track real callback times we run into two issues:

* We can potentially fall into double-rate vsyncs due to error in thread
wakeup latency. For instance, we only correct up to 1.5 ms when wakeup
latency can be up to 5 ms in duration. To correct for this error, we
need a sanity check that callback timings won't fall into double-rate
vsyncs.
* Registering and unregistering event listeners can occur rapidly within
the same vsyncs, so that without any correction we can initially
double-fire events. If we only rely on inferring the last event time,
then we may end up skipping a frame, so we'll require the calling
thread registering and unregistering listeners to best-effort cache the
lastest callback time.

Bug: 124383894
Test: systrace
Change-Id: I7bc8f00502a3b798073a77c5ba4794c12cd14341