Speed-up KXmlSerializer with features.html#indent-output

XmlSerializerPerfTest shows that it now only takes < 10% performance
penalty to indent the output.

Bug: 230007772
Test: atest  CorePerfTests:android.libcore.XmlSerializerPerfTest
Change-Id: I3b9769aaf2600b91ec1d1a2811e98fd57f0c7e96
1 file changed