Copy the out/soong/build_number.txt file into sdk snapshots

This change adds the build_number.txt file into the sdk snapshot to
make it easier to debug issues arising in partner builds caused by the
snapshots.

The file is added to the root of the snapshot and is called:
    snapshot-creation-build-number.txt

Bug: 201295663
Test: m nothing
      packages/modules/common/build/mainline_modules_sdks.sh
      # Check the contents of various snapshots.
Change-Id: Ib3ac0cea5c9315cad0b9a3c25d88759097e37163

sdk/update.go

@@ -281,6 +281,10 @@ func appendUniqueVariants(variants []android.SdkAware, newVariant android.SdkAwa
 	return append(variants, newVariant)
 }
 
+// BUILD_NUMBER_FILE is the name of the file in the snapshot zip that will contain the number of
+// the build from which the snapshot was produced.
+const BUILD_NUMBER_FILE = "snapshot-creation-build-number.txt"
+
 // SDK directory structure
 // <sdk_root>/
 //     Android.bp   : definition of a 'sdk' module is here. This is a hand-made one.
@@ -479,6 +483,9 @@ be unnecessary as every module in the sdk already has its own licenses property.
 
 	bp.build(pctx, ctx, nil)
 
+	// Copy the build number file into the snapshot.
+	builder.CopyToSnapshot(ctx.Config().BuildNumberFile(ctx), BUILD_NUMBER_FILE)
+
 	filesToZip := builder.filesToZip
 
 	// zip them all