logcat: clear when specifying file output

If -c and -f are both specified, clear the files rather
than notifying the logger to clear its data. The -f then
acts like a switch between clearing the in-memory log
data, or the on-disk logrotate data.

Bug: 28936216
Change-Id: Ib1d1fb46ea09f81a2fd9bebb6c8f0f9f2355f6e8