Add android.Expand

Add android.Expand to expand $() variables in properties.

Test: expand_test
Bug: 31948427
Change-Id: Id30856a1d21d02e8997fcf2358e4a5feeede05be

Android.bp

@@ -50,6 +50,7 @@ bootstrap_go_package {
         "android/config.go",
         "android/defaults.go",
         "android/defs.go",
+        "android/expand.go",
         "android/hooks.go",
         "android/makevars.go",
         "android/module.go",
@@ -66,6 +67,7 @@ bootstrap_go_package {
         "android/env.go",
     ],
     testSrcs: [
+        "android/expand_test.go",
         "android/paths_test.go",
         "android/prebuilt_test.go",
     ],