[gallery] roll gallery to 6a8d738c94d0710e229d726729c09fdb5ccaf7ed

Incorporating:

* https://github.com/flutter/flutter/issues/120260

* add asset manifest bin loading and asset manifest api

* use new api for image resolution

* remove upfront smc data casting

* fix typecasting issue

* remove unused import

* fix tests

* lints

* lints

* fix import

* revert image resolution changes

* Update image_resolution_test.dart

* Update decode_and_parse_asset_manifest.dart

* make targetDevicePixelRatio optional

* Update packages/flutter/lib/src/services/asset_manifest.dart
Co-authored-by: Jonah Williams <jonahwilliams@google.com>

* Update packages/flutter/lib/src/services/asset_manifest.dart
Co-authored-by: Jonah Williams <jonahwilliams@google.com>

* fix immutable not being imported

* return List in AssetManifest methods, fix annotation import

* simplify onError callback

* make AssetManifest methods abstract instead of throwing UnimplementedError

* simplify AssetVariant.key docstring

* tweak _AssetManifestBin docstring

* make AssetManifest and AssetVariant doc strings more specific

* use List.of instead of List.from for type-safety

* adjust import

* change _AssetManifestBin comment from doc comment to normal comment

* revert to callback function for onError in loadStructuredBinaryData

* add more to the docstring of AssetManifest.listAssets and AssetVariant.key

* add tests for CachingAssetBundle caching behavior

* add simple test to ensure loadStructuredBinaryData correctly calls load

* Update asset_manifest.dart

* update docstring for AssetManifest.getAssetVariants

* rename getAssetVariants, have it include main asset

* rename isMainAsset field of AssetMetadata to main

* (slightly) shorten name of describeAssetAndVariants

* rename describeAssetVariants back to getAssetVariants

* add tests for TestAssetBundle

* nits

* fix typo in docstring

* remove no longer necessary non-null asserts

* update gallery and google_fonts versions

---------
Co-authored-by: Jonah Williams <jonahwilliams@google.com>

See https://github.com/flutter/flutter/issues/93975

This reverts commit 36a0bc5b.

Revert "Reland "Roll Engine from 6993cb229b99 to c217d9d06e4f (60 revisions) (#76352)" (#76394)" (#76403)

This reverts commit 562f53bf.

This reverts commit c310717a.

Also update Gallery to a version that is compatible with this roll.

The previously broken test should be fixed by
https://github.com/flutter/gallery/pull/331

This reverts commit 2ae09caa.

This shouldn't break our tests as
https://github.com/flutter/gallery/issues/275 is closed.

This reverts commit e8252817.