Ensure that DepIsInSameApex is not called for ExcludeFromApexContentsTag

The ExcludeFromApexContentsTag marker interface was added to avoid
every implementation of DepIsInSameApex() from having to deal with the
special tags, like PrebuiltDepTag. Unfortunately, when adding that
not all calls to DepIsInSameApex() were protected which meant that the
BootImageModule, which panics if it doesn't recognize a tag, was
causing failures. This change documents the need and improves the
consistency.

A follow up change will add a test for this.

Bug: 182992071
Test: m nothing
Change-Id: If0bf9a7447ebf7a0bb0c88e91951a7220d4af45c

android/apex.go

@@ -140,9 +140,24 @@ type DepIsInSameApex interface {
 	// DepIsInSameApex tests if the other module 'dep' is considered as part of the same APEX as
 	// this module. For example, a static lib dependency usually returns true here, while a
 	// shared lib dependency to a stub library returns false.
+	//
+	// This method must not be called directly without first ignoring dependencies whose tags
+	// implement ExcludeFromApexContentsTag. Calls from within the func passed to WalkPayloadDeps()
+	// are fine as WalkPayloadDeps() will ignore those dependencies automatically. Otherwise, use
+	// IsDepInSameApex instead.
 	DepIsInSameApex(ctx BaseModuleContext, dep Module) bool
 }
 
+func IsDepInSameApex(ctx BaseModuleContext, module, dep Module) bool {
+	depTag := ctx.OtherModuleDependencyTag(dep)
+	if _, ok := depTag.(ExcludeFromApexContentsTag); ok {
+		// The tag defines a dependency that never requires the child module to be part of the same
+		// apex as the parent.
+		return false
+	}
+	return module.(DepIsInSameApex).DepIsInSameApex(ctx, dep)
+}
+
 // ApexModule is the interface that a module type is expected to implement if the module has to be
 // built differently depending on whether the module is destined for an APEX or not (i.e., installed
 // to one of the regular partitions).
@@ -260,6 +275,10 @@ type ApexProperties struct {
 //
 // Unless the tag also implements the AlwaysRequireApexVariantTag this will prevent an apex variant
 // from being created for the module.
+//
+// At the moment the sdk.sdkRequirementsMutator relies on the fact that the existing tags which
+// implement this interface do not define dependencies onto members of an sdk_snapshot. If that
+// changes then sdk.sdkRequirementsMutator will need fixing.
 type ExcludeFromApexContentsTag interface {
 	blueprint.DependencyTag