add ContextClassLoader interface, allow custom ContextFactory.createClassLoader for Android

[kinpoo]

An example RingoJS on Android at
https://github.com/kinpoo/ringojs-android-example, use
https://github.com/F43nd1r/rhino-android to dynamically load classes on
android/dalvik.

[botic]

Can you provide a more detailed description why this change is needed (with a reference)? I guess Android requires to enforce a context class loader, but since I never developed for this platform, you have to provide further details of this change.

[kinpoo]

Android Runtime(ART) https://en.wikipedia.org/wiki/Android_Runtime is an application runtime environment for Android. ART uses the same input bytecode as Dalvik https://en.wikipedia.org/wiki/Dalvik_(software).

Android application's souce code compiled to java bytecode store as .class files, it can not run on ART directly, it must be translated to Dalvik bytecode .dex files use Android's d8/dx tool first.

Rhino's compiler org.mozilla.javascript.tools.jsc.Main can translate javascript into java bytecode .class, if need run on Android, need override the ContextFactory.createClassLoader to do the translating job.