mac-update: Add retries to file open

Even after moving mac-update much later in the boot sequence, it still
sometimes comes up before the mac data file is available.

Retry opening the file once every 4 seconds for 120 seconds.

Change-Id: Ie8c87808965fc5db5b4748334e62d9a7f1d004ec
Merge "mac-update: Add retries to file open" into android-4.4