Tapping a modal bottom sheet should not dismiss it by default

[darrenaustin]

Description

As pointed out in #20720, our current implementation of the modal bottom sheets aren't following the Material spec, as tapping on a bottom sheet itself will dismiss the sheet. From the Material Guidelines, it should be dismissed by:

• Tapping a menu item or action within the bottom sheet
• Tapping the scrim
• Swiping the sheet down
• Using a close affordance within the bottom sheet’s top app bar, if available

The app would be responsible for the first and last as it would be providing the controls in the sheet. The other two are already handled by the bottom sheet implementation, however we also dismiss on a tap.

This PR removes the GestureDetector responsible for dismissing the sheet when the user taps on it. While this doesn't technically break the API, it does change the behavior of modal bottom sheets to bring it in line with the spec.

In the unlikely event that an application actually wanted this behavior they could wrap their bottom sheet content in a GestureDetector to replicate the previous behavior:

GestureDetector(
   onTap: () => Navigator.pop(context),
   behavior: HitTestBehavior.opaque,
   child: <bottom sheet contents>,
);

Related Issues

Fixes: #20720

Tests

Modified the test to check to see if a tap doesn't dismiss the sheet.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change: announcement

[HansMuller]

LGTM