[Android] Fix a bug in the DetectCoreDirectoryFragment.

Would crash if an unsupported file was tapped twice because it would set the inFileBrowser boolean to false, thus leading to the attempt to launch said unsupported file. Fixes this.

Also made it display a brief toast if no cores support the extension of the file.
Merge pull request #433 from lioncash/master

[Android] Fix a bug in the DetectCoreDirectoryFragment.