bootctrl: Fix vendor space usage to not conflict with theme data.

Pixel writes theme data to the first bytes of vendor space, and reserves
the first 32x32 bytes for other such flags (such as the SOTA flag). To
avoid conflicts, instead use new AOSP-generic methods in
bootloader_message and libboot_control.

Bug: 139156011
Test: manual test
Change-Id: I7f31847acc21a0eeb46640cb9ed70be2a3f9531c
5 files changed