Prebuilt is enabled even if the source one is not.

This fix a bug in a recent android_app_import-related change
(I8215aa1a6dad74b503eaa1169a64f8c30dda287c) where it uses an incorrect
module reference, and so disables a prebuilt module if its source-based
counterpart is disabled.

Test: app_test.go
Bug: 152343232
Change-Id: Ic7f5ca40be58f06e44b5ec4945caf209ef2dae96