Build: Increase system image for generic_arm64

Prebuilts are now preopted. This requires a bump in system image
size. Technically a 13M bump would have been fine, but round up
to 50M for a little bit of future breathing space.

Bug: 17772057
Change-Id: Ib10dc24960c0b2e03ef28c55c3c199382802d4e1