Split CleanSpec processing out of main Kati run

Instead of pairing the reading of CleanSpec.mk files with the reading of
Android.mk files, split them into separate, individually cachable steps.
This way we only read Android.mk files once after a clean/sync. We'll
still read the CleanSpec.mk files multiple times, but that's
significantly faster than reading all the Android.mk files.

This adds about 50ms if kati doesn't need to reread the CleanSpec.mk
files. Reading all the CleanSpec.mk files takes about a second.

Bug: 35970961
Test: m clean; m nothing; m nothing
Test: Add CleanSpec.mk line, see it executed.
Change-Id: I83bad15c50709510959d5b8b673a907b8aa7de82

ui/build/sandbox_darwin.go

@@ -22,11 +22,12 @@ import (
 type Sandbox string
 
 const (
-	noSandbox     = ""
-	globalSandbox = "build/soong/ui/build/sandbox/darwin/global.sb"
-	makeSandbox   = globalSandbox
-	soongSandbox  = globalSandbox
-	katiSandbox   = globalSandbox
+	noSandbox            = ""
+	globalSandbox        = "build/soong/ui/build/sandbox/darwin/global.sb"
+	makeSandbox          = globalSandbox
+	soongSandbox         = globalSandbox
+	katiSandbox          = globalSandbox
+	katiCleanSpecSandbox = globalSandbox
 )
 
 var sandboxExecPath string