Always use 24-hours locale (#1947)

* Always use 24-hours locale
* use en_US_POSIX instead of en_GB
* Update NSDateFormatter locale for messaging tests

Author: Ramesik

Example/Messaging/Tests/FIRMessagingContextManagerServiceTest.m

@@ -32,6 +32,7 @@ @implementation FIRMessagingContextManagerServiceTest
 - (void)setUp {
   [super setUp];
   self.dateFormatter = [[NSDateFormatter alloc] init];
+  self.dateFormatter.locale = [NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"];
   [self.dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
   self.scheduledLocalNotifications = [NSMutableArray array];
   [self mockSchedulingLocalNotifications];

Firebase/Messaging/FIRMessagingContextManagerService.m

@@ -78,6 +78,7 @@ + (BOOL)handleContextManagerMessage:(NSDictionary *)message {
 + (BOOL)handleContextManagerLocalTimeMessage:(NSDictionary *)message {
   NSString *startTimeString = message[kFIRMessagingContextManagerLocalTimeStart];
   NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
+  dateFormatter.locale = [NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"];
   [dateFormatter setDateFormat:kLocalTimeFormatString];
   NSDate *startDate = [dateFormatter dateFromString:startTimeString];