[iOS,Android] Remove JDK8 method references

It is possible to build JFX on iOS and Android by providing a compatibility library that implements missing JDK8 classes for RoboVM and Dalvik. However, if a method that is new to JDK8 is referenced in an existing RoboVM or Dalvik class, then providing compatibility becomes much harder.

Fortunately, there are very few places where this happens in OpenJFX right now.