Fix loginhint usage in multiuser cache

Issue #328 
isUserMisMatch check in AuthenticationContext is not handling loginhint.
request based on userid or loginhint needs to be clearly defined for each method call.

Fix: 
Updated cache look up for loginhint
Added tests for multi user cache.
<a href='#crh-start'></a><a href='#crh-data-%7B%22processed%22%3A%20%5B%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-android/pull/329%23issuecomment-68001098%22%5D%2C%20%22comments%22%3A%20%7B%22General%20Comment%22%3A%20%7B%22html_url%22%3A%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-android/pull/329%23issuecomment-68001098%22%2C%20%22comments%22%3A%20%5B%7B%22body%22%3A%20%22sign%20off.%20merge%20after%20a%20successful%20build.%22%2C%20%22created_at%22%3A%20%222014-12-23T21%3A35%3A52Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/5713915%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/kpanwar%22%7D%7D%5D%2C%20%22title%22%3A%20%22General%20Comment%22%7D%7D%7D'></a>
<a href='https://www.codereviewhub.com/'><img src='http://www.codereviewhub.com/site/github-bar.png' height=40></a>
- [x] <a href='#crh-comment-General Comment'></a> <img src='http://www.codereviewhub.com/site/github-remaining.png' height=16 width=60>&nbsp;<b><a href='https://github.com/AzureAD/azure-activedirectory-library-for-android/pull/329#issuecomment-68001098'>General Comment</a></b>
- <a href='https://github.com/kpanwar'><img border=0 src='https://avatars.githubusercontent.com/u/5713915?v=3' height=16 width=16'></a> sign off. merge after a successful build.


<a href='https://www.codereviewhub.com/AzureAD/azure-activedirectory-library-for-android/pull/329?mark_as_completed=1'><img src='http://www.codereviewhub.com/site/github-mark-as-completed.png' height=26></a>&nbsp;<a href='https://www.codereviewhub.com/AzureAD/azure-activedirectory-library-for-android/pull/329?approve=1'><img src='http://www.codereviewhub.com/site/github-approve.png' height=26></a>&nbsp;<a href='https://github.com/AzureAD/azure-activedirectory-library-for-android/pull/329'><img src='http://www.codereviewhub.com/site/github-refresh.png' height=26></a>
<a href='#crh-end'></a>