Separate uncrypt into two modes

uncrypt needs to be triggered to prepare the OTA package before
rebooting into the recovery. Separate uncrypt into two modes. In
mode 1, it uncrypts the OTA package, but will not reboot the
device. In mode 2, it wipes the /misc partition and reboots.

Needs matching changes in frameworks/base, system/core and
external/sepolicy to work properly.

Bug: 20012567
Bug: 20949086
(cherry picked from commit 158e11d6)
Change-Id: I349f6d368a0d6f6ee4332831c4cd4075a47426ff