[Android] Remove public modifier for some internal classes.

There are some classes which are not exposed for external usage. This fixes
some classes, including:
1. Merge XWalkResourceClientImpl.java and XWalkUIClientImpl.java into
XWalkResourceClient.java and XWalkUIClient.java.
2. Remove the public modifier for XWalkContent. All tests are based on
XWalkView.java.
3. Fix bugs for 'XWalkView.load()' if the data is set.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1481