Cleanup Android assets reader (preparation before doing further changes):
- CFileSystem::addFileArchive must grab the archive, this should also be fixed in trunk. CIrrDeviceAndroid instead has to drop the archive after adding it.
- Kick public static Activity variables which are not necessary (that's basically globals and made code correspondingly harder to understand)
- Remove createAndroidAssetFileArchive function which is not public accessible and not used anywhere.
- Fix indention
- Irrlichtify variable names (locals start lowercase)


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4678 dfc29bdd-3216-0410-991c-e03cc46cb475
- addDirectory renamed to addDirectoryToFileList and now in IFileArchive so users can add any assets folders.
- asset directories which are added are now released again (was leaking before)
- "media" is no longer added automatically to assets folders by the engine, but must be added by users now (example is extended to do that).
- root assets folder no longer added in CAndroidAssetFileArchive but in CIrrDeviceAndroid to avoid calling a virtual function in a constructor (it had worked in this case for now - still was ugly and should never be done)


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4679 dfc29bdd-3216-0410-991c-e03cc46cb475