Teach MethodSignature about default methods
MethodSignature dates back to Java 7 days and has no notion of
interface-default methods. Retrofit the support, as it may come
handy when dealing with various aspects of codegen.
Also propagate the knowledge of them into java-api-generator, so
that we have a cleaner nonnull method generation and new default
methods do not break Builder{Generator,Template} assumptions.
JIRA: MDSAL-18
JIRA: MDSAL-396
Change-Id: I12ea42d096ee50ae61f908e9f9347c45883f66b3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>