Fix comparing Instants that share the same day

This commit fixes an incorrect implementation of comparing two Instants
that are on the same day. My original implementation was turned out to
be buggy. Using `ChronoUnit#between` is the way to go!

app/src/main/java/org/selfconference/android/util/Instants.java

@@ -19,9 +19,7 @@ public static Instant fromEstString(String text) {
   }
 
   public static boolean areOnSameDay(Instant left, Instant right) {
-    Instant leftTime = left.truncatedTo(ChronoUnit.DAYS);
-    Instant rightTime = right.truncatedTo(ChronoUnit.DAYS);
-    return leftTime.compareTo(rightTime) == 0;
+    return ChronoUnit.DAYS.between(left, right) == 0;
   }
 
   public static String dayTimeString(Instant instant) {