Support asynchronous odsign.

To improve boottime, we want to run odsign in an asynchronous fashion;
but there are 2 places where we do need it be sync:
1) We need to know when it's done using its key, so that we lock
keyrings and advance the boot stage
2) We need to know verification is complete before we start the zygote

These are indicated by odsign using separate properties.

Bug: 165630556
Test: init waits for the properties, and proceeds when done
Change-Id: I623c24a683340961b339ed19be2f577d9293b097