New postinstall domain and rules to run post-install program.

When using the A/B updater, a device specific hook is sometimes needed
to run after the new partitions are updated but before rebooting into
the new image. This hook is referred to throughout the code as the
"postinstall" step.

This patch creates a new execution domain "postinstall" which
update_engine will use to run said hook. Since the hook needs to run
from the new image (namelly, slot "B"), update_engine needs to
temporarly mount this B partition into /postinstall and then run a
program from there.

Since the new program in B runs from the old execution context in A, we
can't rely on the labels set in the xattr in the new filesystem to
enforce the policies baked into the old running image. Instead, when
temporarily mounting the new filesystem in update_engine, we override
all the new file attributes with the new postinstall_file type by
passing "context=u:object_r:postinstall_file:s0" to the mount syscall.
This allows us to set new rules specific to the postinstall environment
that are consistent with the rules in the old system.

Bug: 27177071
TEST=Deployed a payload with a trivial postinstall script to edison-eng.

Change-Id: Ib06fab92afb45edaec3c9c9872304dc9386151b4