Add PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES

For GSI targets, this variable is set true, and Soong will install both
"flattened" and "unflattened" APEXes together in /system_ext and /system
respectively.

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

Change-Id: Ib72d1b9dbd59727942da39323ee7e60ac6e14882