Install flattened/unflattend apexes for GSI

GSI targets are supposed to have both 'flattened' and 'unflattened'
APEXes. By adding 'flattened' APEX as REQUIRED moduled for 'unflattened'
APEX, both will be installed togetther.

This is done by a new variable PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES,
which is set only for GSI targets.

Bug: 137802149
Test: lunch aosp_arm64-userdebug && m
  resulting apex images under /system/apex
  and flattened apexes under /system/system_ext/apex

Change-Id: I336e2674e427b358542e0045b2a49dfa3d84095b