[Android] Add XWalkManifestReader.java to xwalk_core_library project.

The Java class XWalkManifestReader provides the content of app
manifest. Missing of XWalkManifestReader will lead to error of
core library project, so add it to xwalk_core_library.

This patch also fixes some coding style issues.
Merge pull request #1042 from gaochun/cordova_xwalk

[Android] Add XWalkManifestReader.java to xwalk_core_library project.