Handle static methods in Plugins better.

Static methods in plugin interfaces are now left as static.

Also, don't generate "Protector" methods for methods that have a
"$" in them - these methods are generated by kapt, but don't compile
successfully in the final code.

Bug: 383569207
Flag: EXEMPT minor refactor
Test: compile code with K2 enabled
Change-Id: Ibeb635e91c75de1e6f456ba95a283a0be2f142d4