LruCache: add a "contains" method

Adds a contains(TKey) method to the LruCache so that users of the class
can see if they'd previously added an entry.

This was previously accomplisehd by "get", but for classes whose null
value is a valid key (such as ints) this is insufficient.

Bug: n/a
Flag: EXEMPT small new function
Test: new tests
Change-Id: I57c51dfe56819ef98e17d05187cff70afb6f8bf6