Repository: flutter
Issue: [android] `SliverAppBar` stretch doesn't function in `CustomScrollView` with `AlwaysScrollableScrollPhysics`
Labels: platform-android,framework,f: scrolling,d: api docs,d: examples,d: stackoverflow,has reproducible steps,P2,found in release: 2.5,found in release: 2.6,team-android,triaged-android

## Steps to Reproduce

``` dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: CustomScrollView(
          physics: const AlwaysScrollableScrollPhysics(),
          slivers: [
            SliverAppBar(
              pinned: true,
              stretch: true,
              flexibleSpace: const FlexibleSpaceBar(
                centerTitle: false,
                title: Text('title'),
              ),
              expandedHeight: 160,
              onStretchTrigger: ()  async => print('stretched'),
            ),
          ],
        ),
      ),
    );
  }
}

```

**Expected results:** 

Just like `RefreshIndicator `, when `AlwaysScrollableScrollPhysics` you'd want the pulldown stretch/refresh to function even when there aren't enough items in the list to create a scroll behaviour. 

**Actual results:** <!-- what did you see? -->

Allow `SliverAppBar` stretch to function in empty lists when `AlwaysScrollableScrollPhysics` is set.


<details>
  <summary>Logs</summary>

```  powershell
[√] Flutter (Channel stable, 2.5.1, on Microsoft Windows [Version 10.0.19043.1237], locale en-GB)
    • Flutter version 2.5.1 at D:\Android\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision ffb2ecea52 (2 days ago), 2021-09-17 15:26:33 -0400
    • Engine revision b3af521a05
    • Dart version 2.14.2

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at D:\Android\
    • Platform android-30, build-tools 29.0.2
    • ANDROID_HOME = D:\Android
    • Java binary at: D:\Android\openjdk\bin\java
    • Java version OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_202-b08)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Build Tools 2019 16.11.2)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools
    • Visual Studio Build Tools 2019 version 16.11.31624.102
    • Windows 10 SDK version 10.0.19041.0

[!] Android Studio (not installed)
    • Android Studio not found; download from https://developer.android.com/studio/index.html
      (or visit https://flutter.dev/docs/get-started/install/windows#android-setup for detailed instructions).

[√] Connected device (4 available)
    • GM1925 (mobile)   • 686db41d • android-arm64  • Android 10 (API 29)
    • Windows (desktop) • windows  • windows-x64    • Microsoft Windows [Version 10.0.19043.1237]
    • Chrome (web)      • chrome   • web-javascript • Google Chrome 93.0.4577.82
    • Edge (web)        • edge     • web-javascript • Microsoft Edge 93.0.961.47

! Doctor found issues in 1 category.
```

</details>