[layertracegenerator] stream layer trace to file

Write the generated layer trace entry by entry
so we don't hold the entire unserialized layer trace
in memory.

Fixes: 272158947
Test: presubmit
Test: open streamed trace in winscope
Test: cat /proc/`pidof layertracegenerator`/status
old
VmHWM:	  959496 kB
new
VmHWM:	   21416 kB

Change-Id: I4bf0cd8fe98d1e2a44fb6b7c85c4e06e586a52ed