fix: #1472 Stop on boarding next button to get spammed

[AshAman999]

What

• Make the dissimissble to false for dialog if intended to,
• Implementation of one of the solutions as suggested in the comments,

Screenshot

Record_2022-04-22-18-17-10_cc729dbb1ea2c751a543590b5da9708c.mp4

Fixes bug(s)

• Onboard: the first Next button can be spammed #1472

[codecov-commenter]

Codecov Report

Merging #1611 (c45325a) into develop (2ea0da3) will decrease coverage by 0.13%.
The diff coverage is 1.50%.

@@            Coverage Diff             @@
##           develop   #1611      +/-   ##
==========================================
- Coverage     8.86%   8.72%   -0.14%     
==========================================
  Files          161     164       +3     
  Lines         6623    6782     +159     
==========================================
+ Hits           587     592       +5     
- Misses        6036    6190     +154     

Impacted Files	Coverage Δ
...rds/knowledge_panels/knowledge_panels_builder.dart	2.17% <0.00%> (ø)
...s/product_cards/smooth_product_card_not_found.dart	0.00% <0.00%> (ø)
.../smooth_app/lib/data_models/onboarding_loader.dart	0.00% <ø> (ø)
...mooth_app/lib/data_models/product_query_model.dart	1.38% <ø> (ø)
...s/smooth_app/lib/data_models/user_preferences.dart	34.42% <ø> (+2.11%)	⬆️
...s/smooth_app/lib/database/dao_string_list_map.dart	0.00% <0.00%> (ø)
...ckages/smooth_app/lib/database/local_database.dart	0.00% <0.00%> (ø)
...s/smooth_app/lib/database/paged_product_query.dart	0.00% <0.00%> (ø)
...oth_app/lib/database/robotoff_questions_query.dart	0.00% <0.00%> (ø)
...ric_lib/buttons/smooth_large_button_with_icon.dart	0.00% <0.00%> (ø)
... and 16 more

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[g123k]

LGTM

[teolemon]

merged, thanks @AshAman999