bootloader_message: Add helpers for handling IBootControl MergeStatus.

Move merge_status from bootloader_control_ab, which is in vendor space,
to a new generic AOSP struct in system space. This will allow more
devices to share the same HAL implementation.

This patch also changes libboot_control to compensate for merge_status
moving out of vendor space. The reference HAL library now also provides
separate helper functions for managing the merge status, so devices
using a custom boot control HAL can still take advantage of the new misc
implementation.

Bug: 139156011
Test: manual test
Change-Id: I5cd824e25f9d07aad1476301def5cdc3f506b029