Repository: flutter
Issue: TextField labelText doesn't get hintTextStyle when labelTextStyle is non-null
Labels: a: text input,framework,f: material design,has reproducible steps,found in release: 2.2,found in release: 2.5,team-design,triaged-design

## Steps to Reproduce

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1. You can have a look at this little reproduction [https://dartpad.dev/?id=b401619acd87ab339da95a0c8e216e79&null_safety=true](https://dartpad.dev/?id=b401619acd87ab339da95a0c8e216e79&null_safety=true)

**Expected results:** <!-- what did you want to see? -->
[The documentation for InputDecoration](https://api.flutter.dev/flutter/material/InputDecoration/hintStyle.html) states that a TextField's `labelText` should get the `hintTextStyle` applied when the label is sitting on top of the input.
In the reproduction case, I expect the TextField label to be blue while TextField doesn't has focus or value.

**Actual results:**
`labelText` gets `labelTextStyle` values applied even while sitting on top of the input


I also had a quick look into TextField source, and found that in InputDecorator, `labelTextStyle` is merged into `inlineLabelStyle`, which explains why the `labelText` color always prevails. https://github.com/flutter/flutter/blob/f4abaa0735eba4dfd8f33f73363911d63931fe03/packages/flutter/lib/src/material/input_decorator.dart#L2209-L2211

What's moreover, the `inlineStyle` doesn't seem to be taking `hintTextStyle` into account at all
https://github.com/flutter/flutter/blob/f4abaa0735eba4dfd8f33f73363911d63931fe03/packages/flutter/lib/src/material/input_decorator.dart#L2106-L2109

<details>
  <summary>Logs</summary>

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```
[√] Flutter (Channel stable, 2.2.3, on Microsoft Windows [Version 10.0.19042.1165], locale en-US)
    • Flutter version 2.2.3 at D:\flutter
    • Framework revision f4abaa0735 (7 weeks ago), 2021-07-01 12:46:11 -0700
    • Engine revision 241c87ad80
    • Dart version 2.13.4

[√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
    • Android SDK at C:\Users\siklo\AppData\Local\Android\Sdk
    • Platform android-31, build-tools 31.0.0
    • ANDROID_HOME = C:\Users\siklo\AppData\Local\Android\Sdk
    • Java binary at: C:\Program Files\Java\jdk1.8.0_211\bin\java
    • Java version Java(TM) SE Runtime Environment (build 1.8.0_211-b12)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • CHROME_EXECUTABLE = C:\Program Files\Google\Chrome\Application\chrome.exe

[!] 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).

[√] VS Code (version 1.59.0)
    • VS Code at C:\Users\siklo\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.25.0

[√] Connected device (2 available)
    • Chrome (web) • chrome • web-javascript • Google Chrome 92.0.4515.131
    • Edge (web)   • edge   • web-javascript • Microsoft Edge 92.0.902.73

! Doctor found issues in 1 category.
```

</details>