[Code health] Rename `on*` functions

We have some functions throughout the app that are called `on...Result...` `on...Click...` and similar. These names are really opaque and do not convey anything about the actual behavior that's happening when the function is called. It'd be better to convey semantics in these names instead of intended/current call site use, e.g. `onTakePhoto` --> `startPhotoCapture`, `onSignInButtonClick` --> `attemptUserSignIn` etc.