Replace BiMap with a HashMap

[macgills]

Summary:

BiMap is unnecessary and can be deleted, contansKey(V v) is replaceable by containsValue

[gupta-anmol]

@macgills Can I work on this?

I have identified the following changes that will have to be made:

1. Change BiMap to HashMap in DescriptionsAdapter.java
2. Change BiMap to HashMap in SpinnerLanguagesAdapter.kt
3. Delete BiMap.java (I am not sure, maybe we can keep it)

I didn't find any usage of containsKey(V v), so I don't think there's any issues here.

[macgills]

There is one instance where you will have to use containsValue I think. Please delete BiMap in your PR