#275 MvxCachingFragmentCompatActivity GetLastFragmentInfo() does not …

[markverkiel]

…return the last added fragment.

• Removed unnused usings
• Removed double casting
• Used .ToFragment() extension method.
• Created _lastFragmentTag
• Filling _lastFragmentTag within ShowFragment if Fragment is cacheable

This way GetLastFragmentInfo doesn't rely on SupportFragmentManager.Fragments ArrayList anymore.

After the first time that ShowFragment has been called, and a Cacheable fragment is displayed, _lastFragmentTag is filled with its last shown fragment tag.