adbd: fix remount on non-system-as-root devices.

The system image is now a root image, even on devices that don't use
system-as-root. Fix remount to always remount /, to account for this.

Bug: http://b/115751838
Test: `adb remount; adb shell mount | grep " / "`
Change-Id: I2d89a795bd0bc53af33fe69c9d049bbb64d26f3b
1 file changed