Fix LingerMonitor notification rate limiting

If time since boot is lower than the rate limit, notifications would not
be shown.
This is causing tests to fail on continuous testing.

Test: atest FrameworksNetTests
Bug: b/117516272
Change-Id: I03da28f2ca61119fa0ef9534bb4ce3f6406c1ff2

services/core/java/com/android/server/connectivity/LingerMonitor.java

@@ -90,6 +90,8 @@ public class LingerMonitor {
         mNotifier = notifier;
         mDailyLimit = dailyLimit;
         mRateLimitMillis = rateLimitMillis;
+        // Ensure that (now - mFirstNotificationMillis) >= rateLimitMillis at first
+        mFirstNotificationMillis = -rateLimitMillis;
     }
 
     private static HashMap<String, Integer> makeTransportToNameMap() {