Enable Gradle file system watching

This speeds up incremental builds as daemon watches file changes
as files are modified avoiding I/O during the build command.

Note, this feature is disabled in busytown because our builders
use an OS that is too old to support this feature.

Test: Compared enabled and disabled feature
      running ./gradlew core:core:assembleDebug when modifying
      ViewCompat.java file
      Locally incremental builds went from 5s -> 4s on my P920
Change-Id: I9ce8c8355a8227adee3ec62635e258da784c3660
diff --git a/gradle.properties b/gradle.properties
index 355eed5..4fbcfec 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -3,6 +3,8 @@
 org.gradle.configureondemand=true
 org.gradle.parallel=true
 org.gradle.caching=true
+org.gradle.vfs.watch=true
+
 android.builder.sdkDownload=false
 android.uniquePackageNames=false
 android.enableAdditionalTestOutput=true