[android] list layers sources etc

[ivovandongen]

Fixes #7285

Fixes:

• Layer ownership on remove (no more crashes when accessing a handle to a layer after removing it from the style)

Adds:

• getLayers
• addLayerAbove
• addLayerAt
• removeLayerAt
• getSources

Other changes:

• removeLayer now returns Layer handle
• removeSource returns Source handle

[mention-bot]

@ivovandongen, thanks for your PR! By analyzing this pull request, we identified @jfirebaugh, @zugaldia and @incanus to be potential reviewers.