MainActivity: use System.currentTimeMillis

A little better way to get the ms since epoch.

app/src/main/java/it/danieleverducci/lunatracker/MainActivity.kt

@@ -454,7 +454,7 @@ class MainActivity : AppCompatActivity() {
         var duration = event.quantity
 
         fun isValidTime(timeSeconds: Long, durationSeconds: Int): Boolean {
-            val now = Calendar.getInstance().time.time / 1000
+            val now = System.currentTimeMillis() / 1000
             return (timeSeconds + durationSeconds) <= now && durationSeconds < (12 * 60 * 60)
         }
 
@@ -488,7 +488,7 @@ class MainActivity : AppCompatActivity() {
         durationPlus5Button.setOnClickListener { adjust(5) }
 
         durationNowButton.setOnClickListener {
-            val now = Calendar.getInstance().time.time / 1000
+            val now = System.currentTimeMillis() / 1000
             val start = pickedDateTime.time.time / 1000
             if (now > start) {
                 duration = (now - start).toInt()