libui: rewrite Region with FatVector

Region is heavily used by SurfaceFlinger and the Android animation
framework, but it uses the libutils Vector internally. libutils Vector
allocates four elements via malloc() regardless of the contents of the
Vector. However, Region is overwhelmingly a single-element Vector
(occasionally two), which can be allocated on the stack easily enough.

FatVector is an existing wrapper around std::vector that allows for a
stack-allocated vector when the contents are small enough. Using
FatVector in Region instead of libutils Vector avoids all of the
malloc/free overhead, which improves SurfaceFlinger per-frame runtime
by ~4%.

This CL moves FatVector from libhwui to make it usable by Region.

Test: boots, works, passes tests
Bug: 149096186

Change-Id: I265c6c831bc82f31afe0d100aec2f98344f2390b