Increase timeout when uncrypting OTA

Larger OTAs (750 MB tested) are taking 3-4 minutes
to write, due to the O_SYNC needed to ensure that
the data is actually committed to disk prior to
reboot.

Bug: 18750317
Change-Id: Idfab3ffd0276df4548f69a09c72ad6f4a344b6e6
(cherry picked from commit 01c06dfb)