Declare the phone build step for Dev Updater

Dev Updater serves this app's APK to the phone. Carrying the build step
here rather than in that machine's config keeps it with the project that
knows it, and means a second build machine doesn't have to work it out
again.

package and label are declared too, so the project can be added from a
fresh clone with nothing built: without them there is no APK to read an
identity out of, and adding it is the prerequisite for running the build
step that would produce the first one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012iK6pJhPvKjSFwxK4DkCqM

app/.dev-updater.toml

@@ -0,0 +1,5 @@
+package = "com.example.aiapp"
+label = "AI Sessions"
+
+[prepare]
+command = ["./build-apk.sh"]