Bazel: Increase severity of DefaultCharset to ERROR

All existing instances of this issue have been fixed in preceding
commits. Increase its severity to ERROR so that it is easier to
detect reoccurences.

Change-Id: I50f95152857baac5ca44c15493f23b6f9cfac1bf
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

tools/BUILD

         "-Xep:CannotMockFinalClass:WARN",
         "-Xep:ClassCanBeStatic:WARN",
         "-Xep:ClassNewInstance:WARN",
-        "-Xep:DefaultCharset:WARN",
+        "-Xep:DefaultCharset:ERROR",
         "-Xep:DoubleCheckedLocking:WARN",
         "-Xep:ElementsCountedInLoop:WARN",
         "-Xep:EqualsHashCode:WARN",