Enable unsigned-integer-overflow

Turn on unsigned-integer-overflow for libstagefright.so. This is only
for the top level Android.mk and doesn't turn on overflow checking in
all the libaries and codecs included.

This causes unsigned integer overflows to abort the process instead of
potentially being security bugs.

Bug: 23110888
Change-Id: I719ae48c4c3f940a9431d173621885bd344a8401
(cherry picked from commit 70794607)