fix(SubscriptionsFragment): update last feed time to last video time

With the introduction of the local feed extraction, which only refreshes
the feed naturally once a day, it is possible for the user to update the
`lastFeedWatchedTime`, but not fetch the feed itself, leading to a
disconnect between both values.
When the feed is then later on refreshed, the caught-up-indicator is
only displayed for the videos since the last app open, and not all new
videos since the last refresh.

app/src/main/java/com/github/libretube/helpers/PreferenceHelper.kt

@@ -5,7 +5,6 @@ import android.content.SharedPreferences
 import androidx.core.content.edit
 import androidx.preference.PreferenceManager
 import com.github.libretube.constants.PreferenceKeys
-import java.time.Instant
 
 object PreferenceHelper {
     /**
@@ -105,8 +104,8 @@ object PreferenceHelper {
         return getString(PreferenceKeys.LAST_STREAM_VIDEO_ID, "")
     }
 
-    fun updateLastFeedWatchedTime() {
-        putLong(PreferenceKeys.LAST_WATCHED_FEED_TIME, Instant.now().epochSecond)
+    fun setLastFeedWatchedTime(time: Long) {
+        putLong(PreferenceKeys.LAST_WATCHED_FEED_TIME, time)
     }
 
     fun getLastCheckedFeedTime(): Long {

app/src/main/java/com/github/libretube/ui/fragments/SubscriptionsFragment.kt

@@ -380,7 +380,7 @@ class SubscriptionsFragment : DynamicLayoutManagerFragment(R.layout.fragment_sub
         binding.subFeed.adapter = feedAdapter
         binding.toggleSubs.text = getString(R.string.subscriptions)
 
-        PreferenceHelper.updateLastFeedWatchedTime()
+        PreferenceHelper.setLastFeedWatchedTime(feed.first { !it.isUpcoming }.uploaded )
     }
 
     @SuppressLint("SetTextI18n")