[Android] Introduce reflection layer generator

Generator is a java project which depends on xwalk core internal
to get class definitions.
When executing Generator, it will read the class from specified
list and generate wrapper/bridge class of it based on annotation
XWalkAPI.
The bridge class belongs to internal package, provides two functions
for each API internal class wants to expose: wrapperCall and superCall.
wrapperCall is to call wrapper which might be overriden by embedders.
superCall is to call internal which is called by wrapper for default
implementation.
The wrapper class belongs to core package, provides API to embedders.
Wrapper class will call bridge's superCall by default.
If no overridden by embedders, the call stack will be:
bridge::foo => wrapper::foo => bridge::fooSuper => internal::foo.

TODO:
1. Documentation annotation support, currently all documentation annotation
is in internal class. Need to automatically generate those annotation in
wrapper class.
2. GYP dependency, in gyp world, each java target will add its output to
jars_input_paths of dependent target. For reflection layer, it depends
on core_internal, but in future after real reflection established, the
shared embedders don't want to include core internal in it's APK.