Add repositories.properties file

This file will be read by the buildSrc gradle plugin to add
additional local maven repositories.

This CL alone does not have any effect, as buildSrc expects the file
to be next to gradlew. A manifest change would follow this to add
a symlink at that location.

Test: Manually tested Robolectric test runs with other local changes
Bug: 32975598
Change-Id: I100b65b44ac0133a57014f7813967911db1b278c

tools/gradle/repositories.properties

@@ -0,0 +1,3 @@
+repositories {
+    maven { url "$rootDir/prebuilts/maven_repo/robolectric" }
+}