SF: Track missed frames and optionally drop them

Adds code to track whether SurfaceFlinger has sent two frames to HWC in
the same vsync window. This can occur if one frame is delayed so far
it slips into the next window or just if one frame takes an abnormal
amount of time. If this occurs, it shows up as FrameMissed in systrace.

Also adds a property debug.sf.drop_missed_frames which, if set, tells
SurfaceFlinger to skip sending a frame to HWC (i.e., calling
prepare/set) when we detect this condition, which can help prevent
backpressure from the HWC implementation.

Bug: 22513558
Change-Id: I2df0d44cec5fd6edba419388d8c90b5710d1a5b6