Optimize xml serialization path.

Was: 67ms, 20ms, 12ms, 9ms, 107ms, 73ms, 52ms, 26ms, 960ms, 58ms, 28ms, 47ms
Now: 16ms, 34ms, 12ms, 90ms, 101ms, 51ms, 38ms, 23ms, 21ms, 27ms, 36ms, 70ms

hw config:
 CPUs - 2
 GPU Mode - Gfxstream (Accelerated Host GPU Rendering)
 RAM - 2048 mb

Bug: 220083428
Test: presubmit
Change-Id: Ica4e70539a053dfcfd6cb3c0542a33d21ba5b9ff