[TIMOB-10091] Android: Window URL contexts never freed after window has closed.

When opening a window with an "url" script, the context created for this window is never released after the window has been closed. This can lead to memory leaks if the script kept global references to values in that context. See the attached app.js and test.js scripts to reproduce this bug. Create a project with both of these files in the Resources folder. 1. Run application and monitor heap with DDMS. 2. Click "Open Window". 3. Hit the back button to close the window. 4. Repeat steps 2 - 3. Watch the heap size and see if it grows. Actual: Heap size continues to grow in size. Expected: The memory allocated in the new context should be released and eventually collected. Heap size should reach a ceiling and remain stable.