Updates for API 26

[phil-lopreiato]

• PIP still probably breaks everything
• Notification channels/other changes
• The Custom Data store seems like a thing we could want to take advantage of
• More accounts changes

https://developer.android.com/about/versions/o/android-8.0.html

[dracco1993]

Man, if we ever get closer to this, I still want to implement #836. 😁

[phil-lopreiato]

If you want to help me find all the ways multiwindow breaks the app (I'm sure there are many), it'll go a long way in identifying the blockers for bumping the SDK version :)

[1fish2]

https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html reminds us that:

• Apps can't access user accounts without explicit user approval (Android 8.0)

[dracco1993]

This is now irrelevant, as #880 bumped us up to version 28.

[phil-lopreiato]

Hopefully nothing major breaks, but we can file further issues as they arise