maint(android): remove magic `KeymanBuildServer` environment variable from TC build configurations

In the past we used the `KeymanBuildServer` environment variable to limit builds of Keyman for Android to Keyman build agents. This variable is used nowhere in our code/build scripts, and its purpose is not documented nor why it has a particular GUID. So we should remove the variable since it only makes things more confusing when trying to figure out differences between agents.

We can use another Keyman specific variable as condition to limit builds to the Keyman build agents instead.

Start by searching the TeamCity build configurations for the KeymanBuildServer environment variable and identify the existing Keyman-specific variable used to restrict Android builds. Done means the obsolete variable and its unexplained GUID are removed, the existing restriction remains effective, and the affected build configurations validate successfully.

Written by the indexing model from the issue text.