FEATURE preserve user profiles when install new version of RTFaceLogin

[JackMostow]

Problem: Installing a new version -- i.e. with new versionName in build.gradle -- deletes the sqlite database used to store user profiles.

RTFaceLogin displays vnum.setText("v" + BuildConfig.VERSION_NAME); at top left of start screen

Solution: Don't change versionName in build.gradle unless you want to forget existing user profiles! Instead, store version in a different named attribute DisplayVersion in build gradle and change code to use it instead of version.
Should also work for RoboTutor.

Eventually may need to prune outdated profiles – uninstalling app won’t do it, but changing version and recompiling will forget all profiles

@madeleinvillegas @rohankulkz does RTFaceLogin sort gallery by last login time? If so, outdated profiles will sink to the bottom of the gallery (out of sight l→ out of mind)

[JackMostow]

Rather than add and manually update a different variable (e.g. DisplayVersion) in build.gradle in order to identify the version, it seems simpler to use the timestamp of the development branch commit to identify the version. (How) can the code get it?