Remove LiveData from PaymentSessionViewModel.

[jaynewstrom-stripe]

Summary

We did a conversion to remove all these from the paymentsheet module, but it's not happened yet for payments core. I noticed this recently when working on another task.

Motivation

Unify all of our observables around Flow

Testing

Manually verified, and confirmed existing tests pass.

[github-actions]

Diffuse output:

OLD: paymentsheet-example-release-master.apk (signature: V1, V2)
NEW: paymentsheet-example-release-pr.apk (signature: V1, V2)

          │          compressed          │         uncompressed         
          ├───────────┬───────────┬──────┼───────────┬───────────┬──────
 APK      │ old       │ new       │ diff │ old       │ new       │ diff 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
      dex │   3.6 MiB │   3.6 MiB │  0 B │   7.8 MiB │   7.8 MiB │  0 B 
     arsc │   2.3 MiB │   2.3 MiB │  0 B │   2.3 MiB │   2.3 MiB │  0 B 
 manifest │   5.1 KiB │   5.1 KiB │  0 B │  25.4 KiB │  25.4 KiB │  0 B 
      res │ 905.5 KiB │ 905.5 KiB │  0 B │   1.4 MiB │   1.4 MiB │  0 B 
   native │   2.6 MiB │   2.6 MiB │  0 B │     6 MiB │     6 MiB │  0 B 
    asset │     3 MiB │     3 MiB │  0 B │     3 MiB │     3 MiB │  0 B 
    other │ 204.5 KiB │ 204.5 KiB │  0 B │ 460.3 KiB │ 460.3 KiB │  0 B 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
    total │  12.5 MiB │  12.5 MiB │  0 B │  20.8 MiB │  20.8 MiB │  0 B 

 DEX     │ old   │ new   │ diff      
─────────┼───────┼───────┼───────────
   files │     1 │     1 │ 0         
 strings │ 38527 │ 38527 │ 0 (+0 -0) 
   types │ 12669 │ 12669 │ 0 (+0 -0) 
 classes │ 10652 │ 10652 │ 0 (+0 -0) 
 methods │ 55476 │ 55476 │ 0 (+0 -0) 
  fields │ 35667 │ 35667 │ 0 (+0 -0) 

 ARSC    │ old  │ new  │ diff 
─────────┼──────┼──────┼──────
 configs │  292 │  292 │  0   
 entries │ 6936 │ 6936 │  0