Set libavb version into system property for Treble OTA

Set ro.boot.avb_version to "AVB_VERSION_MAJOR.AVB_VERSION_MINOR".
During Treble OTA match, the major version must be the same as that in
the avb metadata on disk, while the minor version can be equal or
greater to that in the avb metadata on disk.

See how avb versioning work on the following link:

  https://android-review.googlesource.com/#/c/342757/

Also renames AvbHashtreeDisabled() -> hashtree_disabled().

Bug: 35322304
Test: Early mount with AVB, checks [ro.boot.avb_version]: [1.0] exists.
Test: Not enable AVB, checks [ro.boot.avb_version] doesn't exists.

Change-Id: I5aaf476ca53c4fe817779518ba14b68ebcfdc6d6