Revert^2 "hwui: remove FatVector"

Revert submission 10449863-revert-fatvector

Reason for revert: b/149254345

Original change (of introducing FatVector) was reverted as a stop-gap solution to fix the aforementioned bug.

The bug was caused by an ABI lock between Surface's definition (that changed with Region) and lib-imsvt prebuilt.

Enabling this change now as we have re-compiled the prebuilt with the change enabled. Doing that via a revert of the revert.

Reverted Changes:
I8ac66acb8:Revert "hwui: remove FatVector"
Ib60dbf3ef:Revert "libui: rewrite Region with FatVector"

Original changes:
I09dc2fddd:hwui: remove FatVector
I265c6c831:libui: rewrite Region with FatVector

bug: 149254345

Change-Id: I9fb5e8908434abb9a763992e922445a2dc37e865