Check if the trace was stopped before starting the tracing thread

If a winscope trace is started and stopped quickly, the tracing thread
will not be stopped. The tracing thread waits for a conditional variable
notification that is sent before the thread starts running. To fix the
issue, properly initialize the trace enabled variable in the tracing
thread.

Test: atest FlickerLibTest:LayersTraceMonitorTest
Fixes: 153206487
Change-Id: I0c591c1fc3209327b40323b3ebb4fa4279c6ed05