Create a `core` module

The aim is to extract the [core package](https://github.com/vector-im/element-android/tree/develop/vector/src/main/java/im/vector/app/core) to its own dedicated module, this will help enable the creation of future modules.

- The core package may contain classes which we don't consider `core` (generic, feature agnostic logic) which we should avoid porting over
- We may want to absorb the [core-utils](https://github.com/vector-im/element-android/tree/develop/library/core-utils) module
- We've agreed to keep the same package name to avoid disruption
- The new module will include Android imports