Thanks to Greg for aligning the type of the `recipients` route param
with this new name, in the recent commit 06a4abc.

Suggested-by: Greg Price <greg@zulip.com>

Instead of a toast with the recipients' names.

When we start targeting Android 12, which we hope to do soon, toasts
will be limited to two lines [1]. It was never a great design to use
a toast here in the first place, since in theory you can have a
group PM conversation with arbitrarily many members, and there's
only so big a toast can get before it starts to look weird or cut
things off in any implementation. With just two lines of space, the
toasts will be even less helpful.

Better to give an action sheet, like we do on long-pressing topic
headers. The action sheet gives a button to take you to the PM
recipients screen, where you can see all the details of the
recipients.

[1] https://developer.android.com/about/versions/12/behavior-changes-12#toast-redesign

Fixes-part-of: zulip#5171

The "failed" part is really important, and we can't have it get cut
off [1] just because a file name is very long.

[1] https://developer.android.com/about/versions/12/behavior-changes-12#toast-redesign

Fixes: zulip#5171

…id 12

See the change described at
  https://developer.android.com/about/versions/12/behavior-changes-12#exported .

A value of "true" means that the component accepts input from other
apps.

- The launcher is another app, so we need "true" there so that Zulip
  can be launched.

- We need "true" for the share-to-Zulip activity; the content to be
  shared originates from other apps.

This change will be required in order to upload new releases to the
Play Store, effective probably 2022-11-01. That isn't very soon, but
we cut it kind of close last time, and we don't want to this time.

The change causes Android 12 and later to apply to our app a number
of behavior changes introduced in that version, documented here:
  https://developer.android.com/about/versions/12/behavior-changes-12

Most of the work for this was zulip#5145, "android notif: Replace our
`NotificationIntentService` with directly opening MainActivity".
Earlier in this series of commits, we also fixed zulip#5171 ("Handle
stricter length limit on Android toasts when targeting Android 12")
and handled the "Safer component exporting" change described at
  https://developer.android.com/about/versions/12/behavior-changes-12#exported .

There were a few other things that stood out in that "Behavior
Changes" article, but none that seem to require any specific action
from us; see
  zulip#5101 (comment) .

Fixes: zulip#5101