[Android][Extension] Move extension tests to core_internal.

ExtensionEcho(renamed as ExtensionEchoInternal) uses core_internal interface
XWalkExtensionAndroid, so this extension along with the test should move
from core into core_internal.

Same thing ExtensionBroadcast.
Merge pull request #2185 from halton/extension_move

[Android][Extension] Move extension tests to core_internal.