Add filter accessor to layer

[ivovandongen]

ATM there is only a setter for filters on Layer classes, no getter, eg Layer#getFilter().

[Jukurrpa]

Hi, I just wanted to know what is the priority on this?

We built most of our design using Mapbox Studio and having access to a layer's filters would be very useful when making changes to it, for example when adding/removing filters on an existing layer while keeping the ones defined through Studio. Storing these base filters in the app or through remote config would somewhat defeat the purpose of Studio.

In the same vein, it'd also be greatly appreciated if layers exposed their source's identifier (in addition to the source layer's identifier).
Edit: same for a VectorSource's list of source layer identifiers

[tobrun]

We are going to add expression filter support to core soon and add Android accessors for it.
This issue has become obsolete.

[1ec5]

We are going to add expression filter support to core soon and add Android accessors for it.

#10720