Introduce cc_api_library

Introduce cc_api_library, which reflects imported API from other
inner-tree. This cc_api_library module type will later refer from
the other ninja module which generates stub library from the interface
description.

Tested environment :
* original libc definition has been removed temporarily, to ensure that
  imported api stub library is being used from build
* Added new definition of libc as below
 cc_api_library {
  name: "libc",
  arch: {
    x86: {
      src: "libs/x86/libc.so",
    },
    x86_64: {
      src: "libs/x86_64/libc.so",
    },
  },
  header_libs: [
    "libc_headers",
  ],
  export_header_lib_headers: ["libc_headers"],
  min_sdk_version: "9",
  vendor_available: true,
 }

Bug: 236087698
Test: `ALLOW_MISSING_DEPENDENCIES=true m vendorimage` succeeded
Change-Id: I67070b0f3561aa2afd73b6c1c0fdf4255218baac

cc/testing.go

@@ -20,6 +20,7 @@ import (
 
 	"android/soong/android"
 	"android/soong/genrule"
+	"android/soong/multitree"
 	"android/soong/snapshot"
 )
 
@@ -31,6 +32,8 @@ func RegisterRequiredBuildComponentsForTest(ctx android.RegistrationContext) {
 	RegisterLibraryHeadersBuildComponents(ctx)
 	RegisterLibraryStubBuildComponents(ctx)
 
+	multitree.RegisterApiImportsModule(ctx)
+
 	ctx.RegisterModuleType("cc_benchmark", BenchmarkFactory)
 	ctx.RegisterModuleType("cc_object", ObjectFactory)
 	ctx.RegisterModuleType("cc_genrule", GenRuleFactory)