Modified revert of commit 6c6f777b.

- The "alphaContactAngleFvPatchScalarField" occurs in several
  places in the code base:

    - as abstract class for two-phase properties
    - in various multiphase solvers

  To resolve potential linking conflicts, renamed the abstract class
  as "alphaContactAngleTwoPhaseFvPatchScalarField" instead.

  This permits potential linking of two-phase and multi-phase
  libraries without symbol conflicts and has no effect on concrete
  uses of two-phase alphaContactAngle boudary conditions.