For AB devices, support flashing two system partitions for factory use.
The normal system image on one partition, but without dex preopt. And a
system_other image that just contains the odex files. The dex files will
not be stripped out of the system image, in case the second system
partition is wiped.

Setting BOARD_USES_SYSTEM_OTHER_ODEX := true in the BoardConfig.mk
enables this behavior.

One can control which directories are placed in system_other by the
SYSTEM_OTHER_ODEX_FILTER configuration variable. Currently we default
to only copying only app and priv-app odexs.

Bug: 29278988
Change-Id: I7f4e87da919e7dc6a89fd8c668193cd4e98631bc