screenrecord: fix race condition

When pressing ctrl-c immediately after starting screenrecord (after
setting the signal handler, but before gStopRequested is set to false)
the record loop would never exit.
Also delete the target file before recreating it, to avoid multiple
instances of screenrecord writing the same file.

Bug: 30247947
Change-Id: I374c125dac69e75638955680a2a5da81e3b22ffe