[Android] Use reflection layer in xwalk core for runtime usage

As now there is reflection layer between xwalk core and xwalk
core internal, there is no need to use the one between runtime
and runtime client for runtime usage.

After this patch, most facilities to support shared mode will be
moved from runtime to xwalk core. Besides, runtime_java will be
merged into runtime_client_java.
XWalkRuntimeLib.apk will keep the name unchanged, but its package
will be org.xwalk.core which was org.xwalk.runtime.lib previously.

TODO: Extension is still using reflection in runtime layer which
will be refined in future commits.
[Android] Move extension manager out to runtime layer

Extension in runtime layer (XWalkExtensionClient) was
directly using XWalkExtensionAndroid, which has core
internal dependency.
It should be based on the extension provide by xwalk
core.
Move all the classes for runtime extension to runtime
client package and remove the reflection layer which
is not between XWalkExtension and XWalkExtensionInternal.
Merge pull request #2387 from wang16/shared_core

[Android] Use reflection layer in xwalk core for runtime usage