Skip contents validation of source bootclasspath fragments

... if prebuilts are selected.

This is a special case of b/361771739 where a apex starts contributing a
jar to the BCP. `disableSourceApexVariant` uses
`PrebuiltSelectionInfoProvider` and this gets populated via the
following dep chain
```
source --> prebuilt --> all_apex_contributions
```

This runs into issues if a prebuilt bootclasspath_fragment does not
exist.

This CL creates a dependency edge between the source module and
all_apex_contributions singleton module so that
`PrebuiltSelectionInfoProvider` is available for
`disableSourceApexVariant`

Bug: 361771739
Test: lunch cf_x86_64-next-userdebug
Test: m nothing with ag/28851886
Change-Id: I92d88199a27579d3c9879d40fd354653370efcce

java/bootclasspath_fragment.go

@@ -463,6 +463,12 @@ func (b *BootclasspathFragmentModule) DepsMutator(ctx android.BottomUpMutatorCon
 	// Add a dependency onto the dex2oat tool which is needed for creating the boot image. The
 	// path is retrieved from the dependency by GetGlobalSoongConfig(ctx).
 	dexpreopt.RegisterToolDeps(ctx)
+
+	// Add a dependency to `all_apex_contributions` to determine if prebuilts are active.
+	// If prebuilts are active, `contents` validation on the source bootclasspath fragment should be disabled.
+	if _, isPrebuiltModule := ctx.Module().(*PrebuiltBootclasspathFragmentModule); !isPrebuiltModule {
+		ctx.AddDependency(b, android.AcDepTag, "all_apex_contributions")
+	}
 }
 
 func (b *BootclasspathFragmentModule) BootclasspathDepsMutator(ctx android.BottomUpMutatorContext) {