Before, updatable_apex.mk was inherited indirectly from
mainline_system.mk. Since mainline_system.mk no longer inherits
updatable_apex.mk, updatable_apex.mk is inherited directly from
device-specific mk file.
Bug: 132664085
Test: choosecombo to phoenix; get_build_var TARGET_FLATTEN_APEX
shows false
Change-Id: I7162567b732f284f618a77cf09f5b40aadd913d5
Merged-In: I7162567b732f284f618a77cf09f5b40aadd913d5