storaged: clean up disk stats code

Merged disk_stats_publisher into disk_stats_monitor class.

Moved proc file check to disk_stats class instead of doing so in
storaged, same for uid_io.

Added is_zero function to check zero disk_stats.

Added operators to compute disk_stats.

Change-Id: I277ae51f6fe18464f92e45c03471343784e67093
diff --git a/storaged/Android.bp b/storaged/Android.bp
index 25b433c..6aef8c8 100644
--- a/storaged/Android.bp
+++ b/storaged/Android.bp
@@ -44,6 +44,7 @@
 
     srcs: [
         "storaged.cpp",
+        "storaged_diskstats.cpp",
         "storaged_info.cpp",
         "storaged_service.cpp",
         "storaged_utils.cpp",