Make mainline device use apex

The build system default was changed to not support apex, but
we want the mainline device to enable it.

Also re-disable it for GSI.

Test: make
Change-Id: I9a4372d375c2c741d7de5b2f83cac3c4b08767f8
Merged-In: I9a4372d375c2c741d7de5b2f83cac3c4b08767f8