Add module based host-tools snapshot

Add new module:
host_snapshot {
    name: "host-snapshot"
    deps: [
      (list of host tools)
    ],
    ...
}

Package host tools using android.PackagingBase to capture host tools and transitive packaging data.
Add JSON meta data to snapshot that allows snapshot to be installed via development/vendor_snapshot/update.py

Add support to generate a fake host snapshot of all host modules that is used to detect required
modules via development/vendor_snapshot/update.py.

Bug: 192556798
Bug: 194799048
Bug: 192896149
Test: m HOST_FAKE_SNAPSHOT_ENABLE=true host-fake-snapshot dist -- check snapshot exists in dist

Change-Id: I849c4db801cd858408f6fe6a3ce69262a23a5be9
diff --git a/android/defs.go b/android/defs.go
index b3ff376..c8e2e9b 100644
--- a/android/defs.go
+++ b/android/defs.go
@@ -188,6 +188,15 @@
 	buildWriteFileRule(ctx, outputFile, content)
 }
 
+func CatFileRule(ctx BuilderContext, paths Paths, outputFile WritablePath) {
+	ctx.Build(pctx, BuildParams{
+		Rule:        Cat,
+		Inputs:      paths,
+		Output:      outputFile,
+		Description: "combine files to " + outputFile.Base(),
+	})
+}
+
 // shellUnescape reverses proptools.ShellEscape
 func shellUnescape(s string) string {
 	// Remove leading and trailing quotes if present