Rename two local variables

Improve code readability by renaming 'device' into 'loop_device' and
'device_fd' into 'loop_fd'.

Bug: 194894000
Test: Built Android images and installed these on an Android device.
Merged-In: Ia9c2d7a525e727f8706e66631b97fc4678c6a4d9
Change-Id: I3fa0c9ca53277b621bb5b81aca394a3079c6e0a3
Ignore-AOSP-First: Already in AOSP.
Signed-off-by: Bart Van Assche <bvanassche@google.com>