snapuserd: Update verification

Initiate update verification from daemon. This will help
in two ways:

1: We avoid reading everything into page-cache. Since,
low end devices are already short on memory, we don't
want to read and populate page-cache which can slow
down boot.

2: During boot, once the selinux transition is done, daemon
is all ready to kick off the verification since verity is
already setup. Note that we are still guarded by update_verifier.
Update_verifier will still block marking new slot as
boot success until the verification is completed. So, there
is no change in the behavior.

Bug: 193863442
Bug: 261913544
Test: Full and incremental OTA on Pixel 6

Incremental OTA of 500M (Monthly OTA)

Boot-time (Without this patch): 38 seconds
Boot-time (With this patch): 32 seconds

Full OTA of 2.2G:

Boot-time (Without this patch): 27 seconds
Boot-time (With this patch): 21 seconds

Signed-off-by: Akilesh Kailash <akailash@google.com>
Merged-In: I4f17db19bdd0dd261902c670be6212862d861fe1
Change-Id: I4f17db19bdd0dd261902c670be6212862d861fe1
8 files changed