Change upload coordinator to not block main thread (#5712)

* Change upload coordinator to not block main thread

Fixes #5707 
Fixes #5708

* Update CHANGELOG.md

Author: mikehaney24

GoogleDataTransport/CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- Stopped GDTCORUploadCoordinator from blocking main thread. (#5707, #5708)
+
 # v6.2.0
 - Added an API for arbitrary data persistence on storage instances.
 - Added an API for fetching storage instances specific to a target.

GoogleDataTransport/GDTCORLibrary/GDTCORUploadCoordinator.m

@@ -63,7 +63,7 @@ - (void)forceUploadForTarget:(GDTCORTarget)target {
  * check the next-upload clocks of all targets to determine if an upload attempt can be made.
  */
 - (void)startTimer {
-  dispatch_sync(_coordinationQueue, ^{
+  dispatch_async(_coordinationQueue, ^{
     self->_timer =
         dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, self->_coordinationQueue);
     dispatch_source_set_timer(self->_timer, DISPATCH_TIME_NOW, self->_timerInterval,
@@ -196,7 +196,7 @@ - (void)appWillForeground:(GDTCORApplication *)app {
 }
 
 - (void)appWillBackground:(GDTCORApplication *)app {
-  dispatch_sync(_coordinationQueue, ^{
+  dispatch_async(_coordinationQueue, ^{
     [self stopTimer];
   });
 }