Update PrimaryButtonUiStateMapper to return state flow.

[jaynewstrom-stripe]

Summary

Continuing to move away from stateIn if we can avoid it. This is for PrimaryButtonUiStateMapper. We can't actually do the combineAsStateFlow in PrimaryButtonUiStateMapper, but this will get us ready to do it.

[github-actions]

Diffuse output:

OLD: identity-example-release-base.apk (signature: V1, V2)
NEW: identity-example-release-pr.apk (signature: V1, V2)

          │          compressed          │         uncompressed         
          ├───────────┬───────────┬──────┼───────────┬───────────┬──────
 APK      │ old       │ new       │ diff │ old       │ new       │ diff 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
      dex │     2 MiB │     2 MiB │  0 B │   4.3 MiB │   4.3 MiB │  0 B 
     arsc │     1 MiB │     1 MiB │  0 B │     1 MiB │     1 MiB │  0 B 
 manifest │   2.3 KiB │   2.3 KiB │  0 B │   8.1 KiB │   8.1 KiB │  0 B 
      res │ 301.5 KiB │ 301.5 KiB │  0 B │   455 KiB │   455 KiB │  0 B 
   native │   7.3 MiB │   7.3 MiB │  0 B │  18.4 MiB │  18.4 MiB │  0 B 
    asset │   1.5 MiB │   1.5 MiB │  0 B │   1.5 MiB │   1.5 MiB │  0 B 
    other │    87 KiB │    87 KiB │ -9 B │ 161.5 KiB │ 161.5 KiB │  0 B 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
    total │  12.2 MiB │  12.2 MiB │ -9 B │  25.8 MiB │  25.8 MiB │  0 B 

 DEX     │ old   │ new   │ diff      
─────────┼───────┼───────┼───────────
   files │     1 │     1 │ 0         
 strings │ 21684 │ 21684 │ 0 (+0 -0) 
   types │  6869 │  6869 │ 0 (+0 -0) 
 classes │  5634 │  5634 │ 0 (+0 -0) 
 methods │ 31446 │ 31446 │ 0 (+0 -0) 
  fields │ 18315 │ 18315 │ 0 (+0 -0) 

 ARSC    │ old  │ new  │ diff 
─────────┼──────┼──────┼──────
 configs │  164 │  164 │  0   
 entries │ 3404 │ 3404 │  0

APK

   compressed    │  uncompressed   │                                           
──────────┬──────┼──────────┬──────┤                                           
 size     │ diff │ size     │ diff │ path                                      
──────────┼──────┼──────────┼──────┼───────────────────────────────────────────
 29.1 KiB │ -4 B │   64 KiB │  0 B │ ∆ META-INF/CERT.SF                        
 25.9 KiB │ -4 B │ 63.9 KiB │  0 B │ ∆ META-INF/MANIFEST.MF                    
    271 B │ -1 B │    120 B │  0 B │ ∆ META-INF/version-control-info.textproto 
──────────┼──────┼──────────┼──────┼───────────────────────────────────────────
 55.2 KiB │ -9 B │  128 KiB │  0 B │ (total)

[jaynewstrom-stripe]

state flows can never complete, so this assert wouldn't pass.

We don't actually rely on this behavior, or use it in our production codepaths, so it's safe to remove.

test also ensure we consumed all the events, so we're safe there.