Adjust README for Flutter Android SDK setup

The README.md suggests the following to be added to the shell config to enable `pidcat`:

```
export PATH=$PATH:/platform-tools
export PATH=$PATH:/tools
```

This step is unnecessary for machines that have the `ANDROID_SDK_ROOT` environment variable set up in the shell config for Flutter developers. We can modify the documentation to account for this since Android Studio also supports Flutter development.

Open README.md and find the pidcat setup instructions that add Android SDK platform-tools and tools to PATH. Compare those instructions with the documented ANDROID_SDK_ROOT shell configuration for Flutter and update the guidance so the unnecessary step is accounted for; done means the README accurately covers this setup.

Written by the indexing model from the issue text.