Use a plain String instead of CharArrayBuffer.

The code used to use CharArrayBuffer to store the numbers as they are
being processed. However, AbstractCursor.copyToCharArrayBuffer()
actually calls AbstractCursor.getString() and we actually instantiate a
new String when comparing the phone numbers: we might as well call
Cursor.getString() directly and avoid creating two new String objects
each time we need to compare numbers.

Empirically, this seems to reduce the run time of the addGroups()
method.

Bug: 5290401

Change-Id: I397a9e6a14657ce261f8b8c11e607b13083bdef1