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
diff --git a/android/variable.go b/android/variable.go
index 863fe5a..b2149c3 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -216,6 +216,7 @@
 
 	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"`