shorebird preview fails to detect app_id when run with an app that uses flavors

[bryanoltman]

Description

#1530 assumes that an app has a single id, which is not true for apps with flavors. Running shorebird preview without specifying an app id in an app with flavors will attempt to use the main app id, which may not have a release.

The fix will likely be to skip app id autodetection if flavors are present.

Steps To Reproduce

1. Run shorebird init --force in the flavors sample project
2. Run shorebird release android --flavor stable
3. Run shorebird preview
4. See "No previewable releases found"