Fix some input device mapping bugs with certain drivers.

On single-touch devices, pointer up/down is signalled by a BTN_TOUCH
key event.  Previously we handled BTN_TOUCH immediately but some drivers
may produce the sequence BTN_TOUCH, ABS_X, ABS_Y, SYN_REPORT on pointer down
which caused us to emit a bad initial pointer down location.
Now we wait for SYN_REPORT before reporting the up or down.

On multi-touch devices, pointer up can be signalled by as little as
the sequence SYN_MT_REPORT, SYN_REPORT.  This change ensures that we
handle this case.

Added support for reading ABS_MT_PRESSURE when available.
Corrected mapping of touchMajor/touchMinor on single touch devices.
Minor code cleanup.

Change-Id: Ic7ec4811241ed85a06e59b8a839ca05180d491d4