Reimplement DepSet as a wrapper around a generic implementation

Implement depSet as a generic depsets implementation using reflection,
and then make DepSet a type-safe wrapper around it.  This will allow
additional wrappers for depsets that work with other types.  All of
this can be replaced with generics once Go supports them.

Test: depset_test.go
Change-Id: Id9df17bcc76f6c1545e7eb498f298066cf8a7679
diff --git a/android/util_test.go b/android/util_test.go
index 25b52ca..fa26c77 100644
--- a/android/util_test.go
+++ b/android/util_test.go
@@ -593,6 +593,10 @@
 			name: "map",
 			f:    firstUniqueStringsMap,
 		},
+		{
+			name: "optimal",
+			f:    FirstUniqueStrings,
+		},
 	}
 	const maxSize = 1024
 	uniqueStrings := make([]string, maxSize)