Add API for writing SF traces to file

Currently the API supports only starting or stopping the trace, and writes to file when the trace is stopped. However, in some scenarios we would like to maintain the trace active, and just dump the trace.

Bug: 205138504
Test: all API, check if files are created under /data/misc/wmtrace
Change-Id: I0102d5f9a96afc159372c5b8ad2261b72f13462f