[Offline map imagery] Add a way to cancel offline maps download

[shobhitagarwal1612]

Looks like the dialog can be dismissed but the tile download still works in the background. We should either make the dialog non-dismissible along with a cancel button or display download progress in the UI or notification bar. Thoughts?

Originally posted by @shobhitagarwal1612 in https://github.com/google/ground-android/issues/1846#issuecomment-1712713459