Ensure SKP serialization occurs on RenderThread

Instead of doing lazy serialization of SKP on the
background executor serialize to a byte[] immediately
at callback invocation. This ensures no potential
for later mutations, race conditions, or wrong-thread issues
at the expense of potentially impacting app rendering performance.

However it seems preferable for a debug-only tool to be a slow
instead of very crashy.

Bug: 141772764
Test: test app
Change-Id: I3316d49970b96f1c59bb0a28ff7335db608e539e