Frameworks/base: Refactor UserHandle and Environment a bit

Refactor slightly to forbid using myUserId in the zygote (uid == 0).
Also factor Environment to put user data (vs shared data) into its
own class, which should keep the page the shared data is on actually
shared between processes.

Bug: 29338430
Change-Id: I05d1306b57658a83299e38076171f56cb364ea80

services/java/com/android/server/SystemServer.java

@@ -284,6 +284,9 @@ public final class SystemServer {
             // we've defined it before booting further.
             Build.ensureFingerprintProperty();
 
+            // Initialize Environment for the system user.
+            Environment.init();
+
             // Within the system server, it is an error to access Environment paths without
             // explicitly specifying a user.
             Environment.setUserRequired(true);