Incremental rollouts of stable app updates to gplay

Should we consider staged rollouts for app updates that roughly match the gradual rollouts used for `server`?

e.g.

* 33% (or whatever) of existing users of 3.25.0 get offered (or, in most cases with gplay, auto-updated to) 3.26.0 initially
* after a policy set period of time, 66% of existing users of 3.25.0 get offered 3.26.0
* after a policy set period of time, 100% of existing users of 3.25.0 get offered 3.26.0
* 100% of existing users of 3.26.0 get offered 3.26.1 when it is released
* 33% of existing users of 3.25.0 (that still haven't updated to 3.26.0) get offered 3.26.1 initially
* etc.

Overrides for security/hot fixes remain would remain options that can always get pushed through on a discretionary basis.

I guess for all I know someone is *already* doing this manually in the app store Play Console at upload time. I'm still wrapping my head around the current release process. It looks like we use Fastlane, but some bits are outside of or still more manual than it seems.

- https://support.google.com/googleplay/android-developer/answer/6346149?hl=en&ref_topic=7072031&sjid=11181899313256575380-NA#
- we can use the `rollout` parameter in Fastlane to do it semi-automatically (fully if we get fancy)