Increase severity of AmbiguousMethodReference to ERROR

All instances of this problem have been fixed. Increase its severity
to ERROR to prevent reoccurrences.

Change-Id: I42d41a7c32b43d1ba59a28cd2f5a7d0ad315d8d9
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

tools/BUILD

         "-Xep:ReferenceEquality:WARN",
         "-Xep:StringEquality:WARN",
         "-Xep:WildcardImport:ERROR",
-        "-Xep:AmbiguousMethodReference:WARN",
+        "-Xep:AmbiguousMethodReference:ERROR",
         "-Xep:BadAnnotationImplementation:ERROR",
         "-Xep:BadComparable:WARN",
         "-Xep:BoxedPrimitiveConstructor:ERROR",