ZooniverseClientTest: Test what the server receives.
Decision tree: Move the files to a sub-directory.
Singleton: Avoid an empty catch to avoid an android-lint warning.
AndroidManifest.xml: WRITE_EXTERNAL_STORAGE is only necessary for sdk version <=18.

If you all you want to do is to write to your own cache. So use maxSdkVersion.
IconsCache: Avoid attempting to create files when we just want to know their path.
IconsCache: Bundle icons from assets and use them if they are new enough.

However, the next commit will make us always use them from assets
and never try to download them, because the CSS file (which describes
which parts of the icons file to use for each icon) changes
for each release, and I don't want to bother with coping wth that,
at least for now.
IconsCache: Only use icons from the bundled assets.

Disable all downloading and caching of icons (and the CSS parsing needed
to do that). Startup is now much quicker, particularly at the start.
I can temporarily revert this commit to download them again so I can
update the bundled assets, though I should really extract the code into
some tool that I can run outside of the app to update the bundled assets.