Proposed workflow steps:

• Adding a page to a reading list remains the way it is today (tap the bookmark icon).
• If the page already exists in a reading list, tapping the bookmark button should show a dialog message saying "Do you want to remove this article from your reading lists?" (yes / no)
• If the user selects Yes, the page is removed from any reading list to which it belongs. (button state adjusts accordingly)
• This should apply to the bookmark button in the bottom toolbar on the page, as well as the bookmark button in the Featured Article feed card.

Change 342225 had a related patch set uploaded (by Dbrant):
[apps/android/wikipedia] Improve logic to remove article from reading list in PageActivity.

https://gerrit.wikimedia.org/r/342225

Proposal based on our discussion today:
If an article appears on 1 list, the bookmark icon is shown as filled with the following actions appearing on the overflow menu:

• Add to reading list
• Remove from <name of list>

Else if an article appears on >1 list, the bookmark icon is shown as filled with the following actions appearing on the overflow menu:

• Add to reading list
• Remove from reading list
  • Selecting this option will bring up a dialog for the user to select which list to remove the article from:

one little request on this proposed dialog: allow selecting all reading lists at once.

Change 342225 merged by jenkins-bot:
[apps/android/wikipedia@master] Improve logic to remove articles from reading lists.

https://gerrit.wikimedia.org/r/342225

Tested on Nexus 4 Android 5.1 and Pixel Android 7.1.1 both running 2.5.195-alpha-2017-04-26

Tested adding and removing articles from single/multiple lists. the behavior appears to be working as desired.