Add a product flag that will enable compressed APEX on device

The product flag is only defined in this CL. No device has been
configured to use this flag yet.

The flag is overridable using environment variable. This way, developers
can enable APEX compression on various targets.

Bug: 137802149
Test: build/soong/soong_ui.bash --dumpvar-mode PRODUCT_COMPRESSED_APEX #false
Test: export OVERRIDE_PRODUCT_COMPRESSED_APEX \
  build/soong/soong_ui.bash --dumpvar-mode PRODUCT_COMPRESSED_APEX #true
Change-Id: I10e4909258cee9b4670bc3f10ff3a0ad5013c864