Trace predictionExpired and dropped frames properly

Allowing frames with prediction expired causes a undesirable skew in the
trace. Since the timestamps are zero for this case, the trace packets
are also sent out with zero timestamp. These zero timestamps are seen as
the earliest possible time in a trace and hence make it look like the
trace contains 1000s of seconds of data while it actually doesn't.
This change addresses this issue by making SurfaceFrames not send any
expected timeline packets if prediction is expired. For actual timeline
packets, we apply a delta to the end time so that it looks like the
frame has a start and an end.

For dropped SurfaceFrames, we don't have an actual end time so far. This
change adds a dropTime to the SurfaceFrame so that we can keep track of
it in trace and dumpsys.

Bug: 179905685
Test: libsurfaceflinger_unittest:FrameTimelineTest

Change-Id: I573037be27f8a74670a65cad791841fa6ed75e20