DO NOT MERGE apex: install flattened from apex_set

apex_set is a set of prebuilt apex files. To support GSI which installs
both flattened and unflattened APEXes when apex_set is used, we extract
the contents from the selected APEX file.

Bug: 159711661
Test: TARGET_PRODUCT=gsi_gms_arm64 m
      should install flattened apexes in /system/system_ext/apex
Change-Id: I0a51b9f4a4537d3230aaa3eece532664ea18bc7c