inputflinger: disable touch input while using a stylus

Android 5.0 cannot form multitouch events from multiple input
devices. So it is not possible to report one touchpoint from the
stylus position and, at the same time, another touchpoint from a
finger touch. Instead, when a new input device starts up the currently
active input is cancelled. This is highly undesirable while writing
with the pen.

The easiest solution is to ignore non-stylus touch events while the
stylus is within range (hovering) of the touchscreen. For example,
N-trig digitizers implement this in hardware. But wacom digitizers do
report pen data simultaneously with touch data. This patch disables
(non-stylus) touch input within 50ms of stylus data. On my Galaxy
Note this is necessary to make stylus input usable.

Original commit by vbraun:
https://github.com/CyanogenMod/android_frameworks_base/commit/b9cb2961309c829586ae24ce432bb1fdb5610ba6

Change-Id: I97f26369826e96c97461c8ae188f1c64dec1b4d3