Rewrite touch navigation dpad synthesis.

The new implementation more accurately tracks the velocity
of flings and takes care to avoid obvious discontinuities.
The main goal is for a fling to appear to be a linear
extension of the movement already in progress.  The minimum
fling velocity is set to ensure that flings appear to be
fairly smooth despite being discretized.

Use sequences of repeated key events instead of individual
down/up events to represent continuous motions in one
direction which can be helpful for stopping flings at boundaries
such as when flinging the cursor position within a text view.

Compute the movement thresholds based on the physical
size of the touch pad, if known.  If not known, we assume a
nominal size.

Support stopping flings with a tap just like we do for
normal touch events elsewhere in the framework.

Moved the detection of ASSIST swipes into the InputReader
where it belongs.  These swipes must be detected globally
to ensure consistent behavior across the all applications.

Added a custom protocol in EventHub to enable input device
drivers to override the timestamp of the following events
in a packet.  This change enables input device drivers
that have a better idea of when an input event was actually
generated to pass this information to the input system.
Particularly useful with uinput.

Bug: 8583760
Change-Id: I8ef4e827804786d549cfaa00793a2b9dd0fda465