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
diff --git a/target/board/BoardConfigMainlineCommon.mk b/target/board/BoardConfigMainlineCommon.mk
index be7c804..be014bf 100644
--- a/target/board/BoardConfigMainlineCommon.mk
+++ b/target/board/BoardConfigMainlineCommon.mk
@@ -41,3 +41,6 @@
 
 # Generate an APEX image for experiment b/119800099.
 DEXPREOPT_GENERATE_APEX_IMAGE := true
+
+# Mainline devices support apex
+TARGET_FLATTEN_APEX := false