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