Really fix crashes with keepalive job with API <= 23 (closes #1619)

Author: schwabe

main/src/main/java/de/blinkt/openvpn/core/keepVPNAlive.java

@@ -90,35 +90,44 @@ public static void scheduleKeepVPNAliveJobService(Context c, VpnProfile vp) {
          * strange Android build that allows lower lmits.
          */
         long initervalMillis = Math.max(getMinPeriodMillis(), 5 * 60 * 1000L);
-        long flexMillis = Math.max(getMinFlexMillis(), 2 * 60 * 1000L);
-        jib.setPeriodic(initervalMillis, flexMillis);
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+            long flexMillis = Math.max(JobInfo.getMinFlexMillis(), 2 * 60 * 1000L);
+            jib.setPeriodic(initervalMillis, flexMillis);
+        }
+        else
+        {
+            jib.setPeriodic(initervalMillis);
+        }
         jib.setPersisted(true);
 
-        JobScheduler jobScheduler = c.getSystemService(JobScheduler.class);
+        JobScheduler jobScheduler = null;
+        jobScheduler = getJobScheduler(c);
+
         jobScheduler.schedule(jib.build());
         VpnStatus.logDebug("Scheduling VPN keep alive for VPN " + vp.mName);
     }
 
-    private static long getMinPeriodMillis() {
+    private static JobScheduler getJobScheduler(Context c) {
+        JobScheduler jobScheduler;
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
-            return JobInfo.getMinPeriodMillis();
+            jobScheduler = c.getSystemService(JobScheduler.class);
+
         } else {
-            return 15 * 60 * 1000L;   // 15 minutes
+            jobScheduler = (JobScheduler) c.getSystemService(JOB_SCHEDULER_SERVICE);
         }
+        return jobScheduler;
     }
 
-    private static long getMinFlexMillis() {
+    private static long getMinPeriodMillis() {
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
-            return JobInfo.getMinFlexMillis();
-        }
-        else
-        {
-           return 5 * 60 * 1000L; // 5 minutes
+            return JobInfo.getMinPeriodMillis();
+        } else {
+            return 15 * 60 * 1000L;   // 15 minutes
         }
     }
 
     public static void unscheduleKeepVPNAliveJobService(Context c) {
-        JobScheduler jobScheduler = c.getSystemService(JobScheduler.class);
+        JobScheduler jobScheduler = getJobScheduler(c);
         jobScheduler.cancel(JOBID_KEEPVPNALIVE);
         VpnStatus.logDebug("Unscheduling VPN keep alive");
     }