We should update our targetSdkVersion to 28, aka Android 9.0 Pie. It's currently 26, since #3075.

Starting 2019-11-01, Google Play will require this for any new updates to our app.

Upstream doc here:
https://developer.android.com/about/versions/pie/android-9.0-migration
and some useful background here:
https://medium.com/androiddevelopers/picking-your-compilesdkversion-minsdkversion-targetsdkversion-a098a0341ebd

The important steps are

1. Read about the potentially-breaking changes, and identify those that might affect us.
2. Make a WIP change to targetSdkVersion.
3. Test the WIP change thoroughly, especially in the areas highlighted in step 1. Fix things as needed, and repeat.

Fortunately this is a less overwhelming set of changes than we had to deal with in #3075 -- because we aren't catching up on 4 years of platform changes at once, as we were then.

More details to fill in later. Filing now in order to have an issue to link to.