Don't reuse StringBuilder outside of lock when dumping ANR.

The ANR dumping code was reusing the shared StringBuilder while
not holding the ActivityManagerService lock.  As a result, other
threads could sweep in and clobber the ANR information.
We don't want to hold the lock here, so just create a new StringBuilder.

Change-Id: I0d91af55f5c123102cfab2cd97035491efed59c0

services/java/com/android/server/am/ActivityManagerService.java

@@ -3181,7 +3181,7 @@ public final class ActivityManagerService extends ActivityManagerNative
         }
 
         // Log the ANR to the main log.
-        StringBuilder info = mStringBuilder;
+        StringBuilder info = new StringBuilder();
         info.setLength(0);
         info.append("ANR in ").append(app.processName);
         if (activity != null && activity.shortComponentName != null) {