Enable sync-c by default

Only fetch the default branch in repo sync
Reduces sync times

Change-Id: I20bcb47c6cd8b18bfb55710f2ce4a60c4c211f7b

default.xml

@@ -6,6 +6,7 @@
            review="https://android-review.googlesource.com/" />
   <default revision="refs/tags/android-8.0.0_r4"
            remote="aosp"
+           sync-c="true"
            sync-j="4" />
 
   <project path="build/make" name="platform/build" groups="pdk" >