Target Android sdk 31 using a fork of reflex-platform supporting this

This changes us to use a fork of reflex-platform with two useful features:

* It accepts arguments controlling the Android sdk versions written to the
  Gradle build file.  We use this feature to bump our target sdk version.
  This will let us build bundles accepted by Google Play Store.

* It has a fix to the Nix expression for Android "Bundle" builds that makes
  the install phase succeed by copying the correct file around.

The latter is not directly relevant to sdk 31 support but it's in the
integration branch so by switching to that branch we're getting it.  This is
great because we'll want it in a minute anyway.