Fix sanitizer in setViewportAndProjection.

On integer sanitized builds calculating b (and possibly t) throws a
runtime error due to an unsigned integer overflow occuring.

 runtime error: unsigned integer overflow: 1024 - 1920 cannot be
 represented in type 'unsigned long'

This changes the type of b (and others) to int32_t, which matches the
types in Rect and can store the negative results above without
overflowing.

Bug: 30969751
Test: Compiles, surfaceflinger tests pass.
Change-Id: Ifb45e3c7e14a2f782412e65d56e462e7df37faba