Handle bad ninepatch data.

Changes proposed by Ben Hawkes of Google Project Zero.

NinePatchPeeker.cpp:
Instead of asserting, return false for bad data.

ResourceTypes.h:
Store ninepatch values as unsigned.

BUG:19151999
Change-Id: Ibe35e7569f632c6bb8a34a7701e26bb6ed547ec2
(cherry picked from commit a730ef3f)