init: Wait for daemon to fully spin up all threads

During selinux transition, daemon will notify `init` process
by writing to file "/metadata/ota/daemon-alive-indicator".

Init will wait until daemon notifies it. Furthermore, daemon
will only write to that file once all threads are spin up
and attached to dm-user misc devices.

Once snapshot-merge is completed, this file will be removed.
Additionally, during boot, init will also ensure that
there are no stale files and will try to remove just
before selinux transition.

Bug: 262407519
Test: OTA on Pixel - Verify new file exits and init waits until daemon
is fully up.
Change-Id: Iabef58ad282d80a7afa493e9df9468ae41a13e44
Signed-off-by: Akilesh Kailash <akailash@google.com>