default.xml: Downgrading to android-11.0.0_r29 until build errors with newer revisions get fixed.

default.xml

@@ -4,7 +4,7 @@
     <remote name="aosp"
             fetch="https://android.googlesource.com"
             review="https://android-review.googlesource.com/"/>
-    <default revision="refs/tags/android-11.0.0_r38"
+    <default revision="refs/tags/android-11.0.0_r29"
              remote="aosp"
              sync-j="4"/>