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
Merged-In: I89eef3e8d06ec1f9d3245513c2dbbff0777e2c62

target/product/gsi_release.mk

@@ -71,3 +71,6 @@ PRODUCT_BUILD_USERDATA_IMAGE := false
 PRODUCT_BUILD_VENDOR_IMAGE := false
 PRODUCT_BUILD_SUPER_PARTITION := false
 PRODUCT_BUILD_SUPER_EMPTY_IMAGE := false
+
+# Always build modules from source
+MODULE_BUILD_FROM_SOURCE := true