Use the public constructure of FastXmlSerializer

FastXmlSerializer() is an @UnsupportedAppUsage. There's another
constructure FastXmlSerializer(int bufferSize) which is public. When
buffer size is zero or negative, the default buffer size will be used.
So FastXmlSerializer() and FastXmlSerializer(0) should be the same.

Bug: 145231333
Test: Manual
Change-Id: I88bab4d655fa0ec59d4445362e06f46b1c5bacec