[Android] Adjust the behavior of loadAppFromManifest and description.

Change the behavior of loadAppFromManifest() in XWalkView.java. Now this
function has the same behavior like load() in XWalkView.java. Besides,
the description of load() and loadAppFromManifest() is refined and more
information is added.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1564