Cleanup SystemInterface.

Make SystemInterface be a Java interface, instead of an abstract base class.
Some of the methods of SystemInterface can be default implemented as they do not depend on a specific
Context being available.
Rename getDefault() to createDefault() as it provides a new object each time it's invoked.

Change-Id: I79ebfdaa667acb5e01372dbd88133b0399f06ed8
Fixes: 66696783
Test: build and flash to home screen, plus runtest com.android.car.CarPowerManagementServiceTest
5 files changed