sun.security.provider.X509Factory: general tidying/linting

- Added generics
- Added @Override annotations
- Cleaned casts
- Use of the constructor for exceptions with cause, instead
of the method initCause
- Use of StringBuilder instead of StringBuffer

Note: this doesn't finish porting the class from jdk8u60, still we
need to port a couple behaviour changes.

Bug: 29631070
Test: make droid docs
Change-Id: I52819f644640d7ee7359e031b44fbc442a2a478d