Update file size formatting.

Per UX, default strings should have space between value and units
resulting in "12.3 GB".  Add a formatting variant that returns the
various components for callers who want to build their own strings.

For now there is only one mounted emulated volume at a time, and
it's always the primary storage, so give it the default rootId to
keep old Uris working.

Change-Id: Ifcc72a91a6b397ee65dc92642153286186eb64ac

core/java/android/net/TrafficStats.java

@@ -53,6 +53,8 @@ public class TrafficStats {
     public static final long GB_IN_BYTES = MB_IN_BYTES * 1024;
     /** @hide */
     public static final long TB_IN_BYTES = GB_IN_BYTES * 1024;
+    /** @hide */
+    public static final long PB_IN_BYTES = TB_IN_BYTES * 1024;
 
     /**
      * Special UID value used when collecting {@link NetworkStatsHistory} for