Java used-by API list generation

Wrap the dexdeps output content with "<externals></externals>" to
simplify backend xml parsing.

Test: TARGET_BUILD_APPS=com.android.adbd m dist apps_only
Change-Id: I1457eb9558763e50911dc9c689b9bc3fb833a52c
Merged-In: I1457eb9558763e50911dc9c689b9bc3fb833a52c
(cherry picked from commit 65a981569f)

scripts/gen_java_usedby_apex.sh

@@ -30,9 +30,11 @@ genUsedByList() {
   shift
   rm -f "$out"
   touch "$out"
+  echo "<externals>" >> "$out"
   for x in "$@"; do
     "$dexdeps" "$x" >> "$out" || true
   done
+  echo "</externals>" >> "$out"
 }
 
 if [[ "$1" == "help" ]]