Java APIs used by Mainline modules.

Add scripts to generate java APIs used by Mainline modules. This is
aosp/1879177 with fixed build error that caused by dexdeps analyzing
resource only APKs which doesn't contains any .dex files. Now skip the
error generates in that case in gen_java_usedby_apex.sh

Forrest run:https://android-build.googleplex.com/builds/abtd/run/L38900000951898586

Test: TARGET_BUILD_APPS=com.android.adbd m dist apps_only
Change-Id: Id539cbe1b7306ace69c047f95ffc02265a467511
diff --git a/apex/testing.go b/apex/testing.go
index 69bd73e..337c862 100644
--- a/apex/testing.go
+++ b/apex/testing.go
@@ -24,6 +24,7 @@
 	android.MockFS{
 		// Needed by apex.
 		"system/core/rootdir/etc/public.libraries.android.txt": nil,
+		"build/soong/scripts/gen_java_usedby_apex.sh":          nil,
 		"build/soong/scripts/gen_ndk_backedby_apex.sh":         nil,
 		// Needed by prebuilt_apex.
 		"build/soong/scripts/unpack-prebuilt-apex.sh": nil,