[Android] Allow selecting an account when sharing to Zulip from other apps

[rajveermalviya]

Currently (after the initial implementation of #53/#52, in #1774) if a user has multiple accounts added and tries to share content from other apps, the sharing page of the first account will be opened unconditionally.

We should instead provide a way to select an account and then show the sharing page of the selected account.

[chrisbobbe]

Currently (after the initial implementation of #53/#52, in #1774)

What happens in this implementation if there aren't any accounts?

[rajveermalviya]

We show an error dialog to the user, asking to login to an account and try again.