Garmin: only add the cache-control header when updating the AGPS file

Prevents it being stale for too long if the user forgets to fetch an updated file.
If the file has to be updated on the watch the response (http status 200) is sent and cached for 14400 seconds, for this period the watch will not request the file again.
Subsequent requests (http status 304) are very small and should not impact battery life negatively, hence they are sent without adding caching directives.

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/service/devices/garmin/agps/AgpsHandler.java

@@ -50,7 +50,6 @@ public class AgpsHandler {
                 }
 
                 final GarminHttpResponse response = new GarminHttpResponse();
-                response.getHeaders().put("cache-control", "max-age=14400");
 
                 final byte[] rawBytes = FileUtils.readAll(agpsIn, 1024 * 1024); // 1MB, they're usually ~60KB
                 final String fileHash = GB.hexdump(CheckSums.md5(rawBytes)).toLowerCase(Locale.ROOT);
@@ -68,6 +67,7 @@ public class AgpsHandler {
                         return response;
                     }
                 }
+                response.getHeaders().put("cache-control", "max-age=14400");
 
                 // Run some sanity checks on known agps file formats
                 final GarminAgpsFile garminAgpsFile = new GarminAgpsFile(rawBytes);