Prefix sysprop for flushing coverage with 'debug.'

Bug: http://b/133322197

Fix selinux denials when reading the coverage.flush sysprop by prefixing
it with 'debug.'.  Sysprops starting with 'debug.' are readable by all
processes when ro.debuggable is set.

Test: Build cuttlefish with coverage and verify that selinux denials for
sysprop reads no longer happen.

Change-Id: I76bef0a658ce881cc81e2d2d4947bef966060376
(cherry picked from commit 9d31ce99069b28d302c5487c2154fc0fd5c2f9fd)