Build against SDKs for native libs only for unbundled apps.

Unbundled APEX modules need to access the platform variant (i.e.
sdk:"") since it's there that the versioned stubs are exposed.

Test: m SOONG_ALLOW_MISSING_DEPENDENCIES=true \
        TARGET_BUILD_UNBUNDLED=true toybox
  on master-art with prebuilt Runtime (Bionic) APEX SDK. The prebuilt
  SDK doesn't have libc.ndk.xxx etc, which e.g. libcrypto would depend
  on without this CL, since it specifies sdk_version.
Bug: 157549171
Change-Id: I3095e42beb2b48421bfb81be942cc2ac30405fd0

android/config.go

@@ -712,10 +712,17 @@ func (c *config) AllowMissingDependencies() bool {
 	return Bool(c.productVariables.Allow_missing_dependencies)
 }
 
+// Returns true if building without full platform sources.
 func (c *config) UnbundledBuild() bool {
 	return Bool(c.productVariables.Unbundled_build)
 }
 
+// Returns true if building apps that aren't bundled with the platform.
+// UnbundledBuild() is always true when this is true.
+func (c *config) UnbundledBuildApps() bool {
+	return Bool(c.productVariables.Unbundled_build_apps)
+}
+
 func (c *config) UnbundledBuildUsePrebuiltSdks() bool {
 	return Bool(c.productVariables.Unbundled_build) && !Bool(c.productVariables.Unbundled_build_sdks_from_source)
 }

android/variable.go

@@ -216,6 +216,7 @@ type productVariables struct {
 
 	Allow_missing_dependencies       *bool `json:",omitempty"`
 	Unbundled_build                  *bool `json:",omitempty"`
+	Unbundled_build_apps             *bool `json:",omitempty"`
 	Unbundled_build_sdks_from_source *bool `json:",omitempty"`
 	Malloc_not_svelte                *bool `json:",omitempty"`
 	Malloc_zero_contents             *bool `json:",omitempty"`

cc/sdk.go

@@ -41,7 +41,7 @@ func sdkMutator(ctx android.BottomUpMutatorContext) {
 			modules[0].(*Module).Properties.Sdk_version = nil
 			modules[1].(*Module).Properties.IsSdkVariant = true
 
-			if ctx.Config().UnbundledBuild() {
+			if ctx.Config().UnbundledBuildApps() {
 				modules[0].(*Module).Properties.HideFromMake = true
 				modules[0].(*Module).Properties.PreventInstall = true
 			} else {