Fix for Gnss Hal for replay

In the old logic, if /dev/gnss0 doesn't report location, it will fail back
to the default mock location. Now we change to only report default location if /dev/gnss0 is
not present.

Test: launch_cvd --start_gnss_proxy
Change-Id: Ie1026d5b4dd3e3c2ed1e2107232bcb94c3ba5079

Change-Id: I787b71d80be7875fc594d17b4296e7d28d9c2f5a