Add buffer-size parameter to FastXmlSerializer ctor

Also increase the default internal buffer size from 8kb to 32kb.  We
have evidence that we're incurring a fair bit of "extra" disk write load
in various common circumstances like SharedPreferences, and this should
help eliminate that.

Bug 63432101
Test: bit CtsContentTestCases:.SharedPreferencesTest

Change-Id: Iba4768262142555a279284b2ae70741424ee2184