file_contexts

The update_engine daemon from Brillo is expected to be used also in
Android so move its selinux policy to AOSP.

Put update_engine in the whitelist (currently only has the recovery
there) allowing it to bypass the notallow for writing to partititions
labeled as system_block_device.

Also introduce the misc_block_device dev_type as update_engine in some
configurations may need to read/write the misc partition. Start
migrating uncrypt to use this instead of overly broad
block_device:blk_file access.

Bug: 23186405
Test: Manually tested with Brillo build.

Change-Id: Icf8cdb4133d4bbdf14bacc6c0fa7418810ac307a