Fix assertion caused by system-initiated process death

Closes #2109 (closed)

Here is how to provoke the crash/assertion on installations that is not running with this fix merged into the code:

1. First navigate to android Settings -> System - Developer options and enable the "Don't keep activities" option (under heading APPS). (I guess the exact location/wording can vary between devices).
2. Start Briar
3. Navigate to Blogs page and tap RSS Feeds (in overflow menu).
4. Plus + to import new feed
5. Type URL https://example.com
6. Tap Import
7. Wait for error dialog.
8. Tap android home button, ◯ (may vary), to show the home screen with all the apps.
9. (the android system will now "not keep" the rss feed import activity, stopping it that is)
10. Tap the "app overview" button, □ (may vary) and navigate (tap) back to Briar.
11. Tap Try Again
12. Enjoy Briad crashed icon.

With the fix, we should obviously not have a crash. Enabling android to not keep activities is the way to quickly provoking this crash, so it is helpful when verifying the fix.