Allow uncrypt to work without socket communication

It was inconvenient to uncrypt a update package under adb shell
because the uncrypt executable required a socket to start its job.
Add a workaround to allow uncrypt executes without socket
communication.

Test: run uncrypt under adb shell, and the block map generates successfully
Bug: 29906218

Change-Id: Ibc328b31636d925dc429ede8dcec7392a721dd53
(cherry picked from commit 28c1e5d3)