Always build modules in GSI from source

GSI includes both flattened and unflattened apex at the same time.
However, a flattend apex is not built if the module is built from
prebuilt binary. So, we enable 'MODULE_BUILD_FROM_SOURCE := true'
to force GSI built all modules from source.

Bug: 194917545
Test: build gsi_gms_arm64-user
Test: check com.google.android.art exists in /system/system_ext/apex/
Change-Id: I89eef3e8d06ec1f9d3245513c2dbbff0777e2c62