Introduce IBootControl 1.1.

libsnapshot needs to communicate to the bootloader that a merge is in
progress. This can be used to prevent factory data resets, prevent
flashing or wiping userdata/metadata, and warning when the active slot
changes.

Bug: 138861550
Test: builds
Change-Id: I577877696b5ec6920b9520d518374931ce9ddfaa