ART: Relayout Region

Relayout the Region fields. Reduces required memory for a Region
object by 20% (80B to 64B). With the default run-test config, this
reduces the Region array size from 164.4KB to 131.8KB.

Bug: 79365543
Test: m test-art-host
Change-Id: Ic044629decd3fabe4e8b964f10fee155ca65abf9