Convert rootDir to string to fix a gradle sync issue

core_settings.gradle

@@ -13,7 +13,7 @@
 // limitations under the License.
 def rootDir = gradle.ext.exoplayerRoot
 if (!gradle.ext.has('exoplayerSettingsDir')) {
-    gradle.ext.exoplayerSettingsDir = new File(rootDir).getCanonicalPath()
+    gradle.ext.exoplayerSettingsDir = new File(rootDir.toString()).getCanonicalPath()
 }
 def modulePrefix = ':'
 if (gradle.ext.has('exoplayerModulePrefix')) {