Surface storage usage of other users.

Bring up separate StorageMeasurement sessions bound to other users
on device, and surface their total usage as teal colors.  Exclude
app code size when measuring secondary users.  Shift preferences to
using keys instead of fragile index ordering.

Bug: 7003520
Change-Id: I214d0b223e53955df71104502596743f049f2027

src/com/android/settings/Settings.java

@@ -103,6 +103,7 @@ public class Settings extends PreferenceActivity
             R.id.device_section,
             R.id.sound_settings,
             R.id.display_settings,
+            R.id.storage_settings,
             R.id.application_settings,
             R.id.personal_section,
             R.id.security_settings,