[Android] Remove public modifier for some internal classes.

There are some internal classes which should not be exposed for external usage.
This patch fixes some of them, 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