Ensure environment dependencies are correct

Store the os Environment on init, then clear it so that it's only
available via a call to Config.Getenv(). That way we're guaranteed to
know about all dependencies.

Test: Add some os.Getenv/Config.Getenv calls and ensure correct behavior
Test: cs/ shows no os.Getenv / os.LookupEnv / os.ExpandEnv calls that
would be affected
Change-Id: Ic519e0c871f340e752df03b8af0599d47c1d91d8

android/config.go

@@ -289,7 +289,7 @@ func (c *config) Getenv(key string) string {
 		if c.envFrozen {
 			panic("Cannot access new environment variables after envdeps are frozen")
 		}
-		val = os.Getenv(key)
+		val, _ = originalEnv[key]
 		c.envDeps[key] = val
 	}
 	return val