MTP: add strict bounds checking for all incoming packets

Previously we did not sanity check incoming MTP packets,
which could result in crashes due to reading off the edge of a packet.
Now all MTP packet getter functions return a boolean result
(true for OK, false for reading off the edge of the packet)
and we now return errors for malformed packets.

Bug: 18113092
Change-Id: Ic7623ee96f00652bdfb4f66acb16a93db5a1c105