Enforce minimum keep alive / poll interval

app/src/main/java/eu/faircode/email/FragmentAccount.java

@@ -1114,7 +1114,7 @@ public class FragmentAccount extends FragmentBase {
                         account.keep_alive_failed = 0;
                         account.keep_alive_succeeded = 0;
                     }
-                    account.poll_interval = poll_interval;
+                    account.poll_interval = Math.max(1, poll_interval);
 
                     account.partial_fetch = partial_fetch;
                     account.ignore_size = ignore_size;

app/src/main/java/eu/faircode/email/FragmentPop.java

@@ -379,7 +379,7 @@ public class FragmentPop extends FragmentBase {
                     account.leave_deleted = leave_deleted;
                     account.leave_on_device = leave_device;
                     account.max_messages = (TextUtils.isEmpty(max) ? null : Integer.parseInt(max));
-                    account.poll_interval = Integer.parseInt(interval);
+                    account.poll_interval = Math.max(1, Integer.parseInt(interval));
 
                     if (!update)
                         account.created = now;