Remove alignment bits in between stack maps

Saves 0.65% of boot.oat size, probably similar on apps. Added
BitMemoryRegion to avoid requiring adding state to StackMap. Added
test to memory_region_test.

Test: clean-oat-host && test-art-host

Bug: 34621054

Change-Id: I40279c59e262bd5e3c6a9135f83e22b5b6900d68