Introduce build variable `OVERRIDE_TARGET_FLATTEN_APEX`.

When `OVERRIDE_TARGET_FLATTEN_APEX` is defined (e.g. set in the
environment), `TARGET_FLATTEN_APEX` is forcibly assigned its value.

This is useful to enable/disable APEX flattening from the command
line (thus ignoring the product's default configuration), for testing
purposes.

Note: Previously it was possible to set `TARGET_FLATTEN_APEX` directly
and have the same effect, but recent changes in the build
configuration now prevent that option.

Test: Check that:
        export OVERRIDE_TARGET_FLATTEN_APEX=true \
          && . ./build/envsetup.sh \
          && lunch aosp_walleye-userdebug \
          && export OVERRIDE_TARGET_FLATTEN_APEX=true \
          && build/soong/soong_ui.bash --dumpvar-mode TARGET_FLATTEN_APEX
      returns:
        true
Bug: 121117762

Change-Id: Ib9ccae38430340de38e4758b4f55df2c65ea60d5