Update timer icons.

Signal/Images.xcassets/ic_timer.imageset/Contents.json

@@ -2,17 +2,17 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "timer-48@1x.png",
+      "filename" : "timer-24@1x.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
-      "filename" : "timer-48@2x.png",
+      "filename" : "timer-24@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "timer-48@3x.png",
+      "filename" : "timer-24@3x.png",
       "scale" : "3x"
     }
   ],

Signal/Images.xcassets/ic_timer_disabled.imageset/Contents.json

@@ -2,17 +2,17 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "timer-disabled-48@1x.png",
+      "filename" : "timer-disabled-24@1x.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
-      "filename" : "timer-disabled-48@2x.png",
+      "filename" : "timer-disabled-24@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "timer-disabled-48@3x.png",
+      "filename" : "timer-disabled-24@3x.png",
       "scale" : "3x"
     }
   ],