[Android] Revert "Make the ResourceExtractor use the App context"

This reverts commit 0f1465d5fae40d3fd29b1c47e367eba488d7ef96.
For xwalk shared mode, the ResourceExtractor needs to read assets via Library Context
and write them to app data via App Context, so it must keep the original Context instead
of getting ApplicationContext of it.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1384
[Android] Address Context leak in ResourceExtractor

The leak is fixed by commit "Make the ResourceExtractor
use the App context"(0f1465d5f), which is reverted
because xwalk shared mode needs to keep original Context.

In this commit, the static instance of ResourceExtractor
will be released after the background job is done. The
ResourceExtractor should not be used any more from that point.