Simplify LinkHandler & source of truth for Link enablement

[samer-stripe]

Summary

Simplify LinkHandler & source of truth for Link enablement

Motivation

Simplify LinkHandler & source of truth for Link enablement

Testing

• Added tests
• Modified tests
• Manually verified

[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 │   2.1 MiB │   2.1 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 KiB │     8 KiB │  0 B 
      res │ 302.9 KiB │ 302.9 KiB │  0 B │   457 KiB │   457 KiB │  0 B 
   native │   6.2 MiB │   6.2 MiB │  0 B │  15.8 MiB │  15.8 MiB │  0 B 
    asset │   7.7 KiB │   7.7 KiB │  0 B │   7.4 KiB │   7.4 KiB │  0 B 
    other │  95.7 KiB │  95.7 KiB │ +4 B │ 183.5 KiB │ 183.5 KiB │  0 B 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
    total │   9.8 MiB │   9.8 MiB │ +4 B │  21.8 MiB │  21.8 MiB │  0 B 

 DEX     │ old   │ new   │ diff      
─────────┼───────┼───────┼───────────
   files │     1 │     1 │ 0         
 strings │ 20668 │ 20668 │ 0 (+0 -0) 
   types │  6493 │  6493 │ 0 (+0 -0) 
 classes │  5259 │  5259 │ 0 (+0 -0) 
 methods │ 31489 │ 31489 │ 0 (+0 -0) 
  fields │ 18222 │ 18222 │ 0 (+0 -0) 

 ARSC    │ old  │ new  │ diff 
─────────┼──────┼──────┼──────
 configs │  164 │  164 │  0   
 entries │ 3646 │ 3646 │  0

APK

   compressed    │   uncompressed   │                                           
──────────┬──────┼───────────┬──────┤                                           
 size     │ diff │ size      │ diff │ path                                      
──────────┼──────┼───────────┼──────┼───────────────────────────────────────────
 29.2 KiB │ +3 B │  64.6 KiB │  0 B │ ∆ META-INF/CERT.SF                        
  1.2 KiB │ -3 B │   1.2 KiB │  0 B │ ∆ META-INF/CERT.RSA                       
 25.9 KiB │ +3 B │  64.5 KiB │  0 B │ ∆ META-INF/MANIFEST.MF                    
    272 B │ +1 B │     120 B │  0 B │ ∆ META-INF/version-control-info.textproto 
──────────┼──────┼───────────┼──────┼───────────────────────────────────────────
 56.5 KiB │ +4 B │ 130.5 KiB │  0 B │ (total)

[samer-stripe]

This has the same condition as isLinkEnabled would after setupLink (linkState != null)

[samer-stripe]

By doing this, we effectively making this handler a logout manager outside of setting up for eager launch. I think there's an opportunity to move Link setup into PaymentElementLoader. From there, we indicate to the individual products if a session is eligible for eager launch through PaymentMethodMetadata then each individual product can decide if they want to launch into it (in this case only PaymentSheet for now)