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

adb/remount_service.c

@@ -113,6 +113,12 @@ void remount_service(int fd, void *cookie)
     char buffer[200];
     char prop_buf[PROPERTY_VALUE_MAX];
 
+    if (getuid() != 0) {
+        WriteStringFully(fd, "Not running as root. Try \"adb root\" first.\n");
+        adb_close(fd);
+        return;
+    }
+
     bool system_verified = false, vendor_verified = false;
     property_get("partition.system.verified", prop_buf, "0");
     if (!strcmp(prop_buf, "1")) {