Touch: slot and tracking updates

Discard type A MT slots if type B MT tracking ID sent: Some devices send
an initial ABS_MT_SLOT event before switching to type B events, so
discard any type A state related to slot.

Track whether ABS_MT_TRACKING_ID was seen in the current sequence to
differentiate between vkeys and hardware keys.

Change-Id: I7ca3bfa4bd82d05ef7f7a208e954898b88bf3865