Fix a FormatFlagsConversionMismatchException bug I introduced in Froyo.

We want to show the characters representing the flags, not the decimal
values of their ASCII representation. "Use overloading judiciously",
as the man says, or cause errors like this.

Seen in FRE56B (see https://android-git.corp.google.com/g/42528).
diff --git a/libcore/luni/src/main/java/java/util/Formatter.java b/libcore/luni/src/main/java/java/util/Formatter.java
index 12561e8..744df1f 100644
--- a/libcore/luni/src/main/java/java/util/Formatter.java
+++ b/libcore/luni/src/main/java/java/util/Formatter.java
@@ -1265,7 +1265,7 @@
             if (strFlags == null) {
                 strFlags = new StringBuilder(7); // There are seven possible flags.
             }
-            strFlags.append(ch);
+            strFlags.append((char) ch);
             return true;
         }