Align PSKKeyManager with framework API style.

This CL adjusts android.net.PSKKeyManager as follows:
* Renamed to PskKeyManager to follow naming conventions.
* Changed from interface to abstract class with default
  implementations for all methods.

Bug: 16403305
Bug: 15073623
Change-Id: Iefce26b394d4a753412315dad554b5342f3f0b44