BLOCK_LABEL_VISIBLE is defined on the wrong interface

Problem/Motivation

In #2029677: Convert BLOCK_LABEL_VISIBLE to a constant on the block interface BLOCK_LABEL_VISIBLE was converted from a constant in block.module to a constant on BlockInterface.
Back then, both BlockPluginInterface and BlockInterface lived in block.module, and since no one noticed that all non-test usages were in a class implementing BlockPluginInterface and not BlockInterface, it was added to the wrong file.

Proposed resolution

Fix this! Leave the old one deprecated. Update the handful of usages.

Remaining tasks

N/A

User interface changes

N/A

API changes

"New" constant on BlockPluginInterface

Data model changes

N/A