Add jitter to job exponential backoff.

app/src/main/java/org/thoughtcrime/securesms/jobmanager/JobController.java

@@ -456,6 +456,9 @@ class JobController {
     int    boundedAttempt     = Math.min(nextAttempt, 30);
     long   exponentialBackoff = (long) Math.pow(2, boundedAttempt) * 1000;
     long   actualBackoff      = Math.min(exponentialBackoff, maxBackoff);
+    double jitter             = 0.75 + (Math.random() * 0.5);
+
+    actualBackoff = (long) (actualBackoff * jitter);
 
     return currentTime + actualBackoff;
   }