check if uid=0 before attempting remount

If "adb remount" is done without having done "adb root" first,
scary looking SELinux denials are emitted before the operation
eventually fails. Avoid the scary looking messages by refusing
remount attempts if we're not running with privileges.

Change-Id: I298621251a10e38345ef77875003a97c8b5a0270
1 file changed