init: set ro.boot.avb_version in recovery mode

Previously we set ro.boot.avb_version during the first stage mount in normal mode:
  - https://android-review.googlesource.com/#/c/371774/

As the first stage mount is not performed in recovery mode, we need to set the
property separately in recovery mode.

Bug: 37414003

Test: first stage mount /vendor with vboot 2.0 (avb) on bullhead in normal mode
Test: first stage mount /system with without verity on bullhead in normal mode
Test: checks ro.boot.avb_version is 1.0 on bullhead in recovery mode

Test: first mount /vendor with with vboot 1.0 on sailfish in normal mode
Test: checks ro.boot.avb_version doesn't exist on sailfish in recovery mode

Change-Id: I262e75b8b557c4de7609b4049ccb01793644245e