Add android_sdk_repo_host to build platform-tools&build-tools

The Android SDK has been built with Make up until now, monolithically,
then split up into several sdk-repo zip files for different purposes.
The Mac and Windows SDKs really only need to contain the platform-tools
and build-tools pieces, but due to this monolithic sdk zip, we had to
build the whole SDK first.

This adds an `android_sdk_repo_host` module that can build these
platform-tools and build-tools zips.

Bug: 187222815
Change-Id: I55809e1d7447dd65e22461f921b2b8abb6d5f822

android/variable.go

@@ -40,6 +40,7 @@ type variableProperties struct {
 		Platform_sdk_version struct {
 			Asflags []string
 			Cflags  []string
+			Cmd     *string
 		}
 
 		Platform_sdk_version_or_codename struct {
@@ -50,6 +51,10 @@ type variableProperties struct {
 			Cmd *string
 		}
 
+		Platform_version_name struct {
+			Base_dir *string
+		}
+
 		// unbundled_build is a catch-all property to annotate modules that don't build in one or
 		// more unbundled branches, usually due to dependencies missing from the manifest.
 		Unbundled_build struct {