Tracking java.lang.reflect.Executable changes

Added a method to Class: total vtable entries increases
by one.

Handling knock-ons of introducing Executable into the
class hierarchy above AbstractMethod.

Rearranging java_lang_reflect_*.cc code to track libcore
changes that reduce duplication between Constructor / Method
/ Executable and AbstractMethod and track the OpenJDK 8
more closely.

Bug: 28666126
Change-Id: I6b5c476a14b8ea25556c35448431de744519b6c2