Revert "use absolute paths in gradle script"

This reverts commit 2df0256.

Author: Mihail Slavchev

build/project-template-gradle/build.gradle

@@ -144,7 +144,7 @@ repositories {
     jcenter()
     // used for local *.AAR files
     flatDir {
-    	dirs '$projectDir/libs/aar'
+    	dirs 'libs/aar'
 	}
 }