Minor cleanup for handling of disabled animations

[Jacalz]

Description:

Just a very simple cleanup PR to move the no_animations tag handling to use existing infrastructure in the internal/build package. The biggest win here is reducing the number of files in the app package but it might theoretically (due to now using a constant for the value) also help the compiler deadcode eliminate more branches when building without animations (although the latter is unlikely to matter in most cases).

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 59.958%. remained the same
when pulling 370d051 on Jacalz:no_animations
into 85063d5 on fyne-io:develop.

[sdassow]

Nice. And always glad to avoid init().