Show a warning for users on API < 19.

We'll be updating minSdk to 19 in 4.37. This lets these users continue
to use the app, but they'll be warned with a persistent banner saying
that they can't receive updates.

res/values/strings.xml

@@ -1397,6 +1397,7 @@
     <string name="reminder_header_share_title">Invite your friends!</string>
     <string name="reminder_header_share_text">The more friends use Signal, the better it gets.</string>
     <string name="reminder_header_service_outage_text">Signal is experiencing technical difficulties. We are working hard to restore service as quickly as possible.</string>
+    <string name="reminder_header_the_latest_signal_features_wont_work">The latest Signal features won\'t work on this version of Android. Please upgrade this device to receive future Signal updates.</string>
 
     <!-- media_preview -->
     <string name="media_preview__save_title">Save</string>

src/org/thoughtcrime/securesms/ConversationListFragment.java

@@ -71,6 +71,7 @@ import org.thoughtcrime.securesms.components.reminder.ServiceOutageReminder;
 import org.thoughtcrime.securesms.components.reminder.ShareReminder;
 import org.thoughtcrime.securesms.components.reminder.SystemSmsImportReminder;
 import org.thoughtcrime.securesms.components.reminder.UnauthorizedReminder;
+import org.thoughtcrime.securesms.components.reminder.UnsupportedAndroidVersionReminder;
 import org.thoughtcrime.securesms.database.DatabaseFactory;
 import org.thoughtcrime.securesms.database.MessagingDatabase.MarkedMessageInfo;
 import org.thoughtcrime.securesms.database.loaders.ConversationListLoader;
@@ -201,6 +202,8 @@ public class ConversationListFragment extends Fragment
         final Context context = params[0];
         if (UnauthorizedReminder.isEligible(context)) {
           return Optional.of(new UnauthorizedReminder(context));
+        } else if (UnsupportedAndroidVersionReminder.isEligible()) {
+          return Optional.of(new UnsupportedAndroidVersionReminder(context));
         } else if (ExpiredBuildReminder.isEligible()) {
           return Optional.of(new ExpiredBuildReminder(context));
         } else if (ServiceOutageReminder.isEligible(context)) {

src/org/thoughtcrime/securesms/components/reminder/UnsupportedAndroidVersionReminder.java

@@ -0,0 +1,23 @@
+package org.thoughtcrime.securesms.components.reminder;
+
+import android.content.Context;
+import android.os.Build;
+import android.support.annotation.NonNull;
+
+import org.thoughtcrime.securesms.R;
+
+public class UnsupportedAndroidVersionReminder extends Reminder {
+
+  public UnsupportedAndroidVersionReminder(@NonNull Context context) {
+    super(null, context.getString(R.string.reminder_header_the_latest_signal_features_wont_work));
+  }
+
+  @Override
+  public boolean isDismissable() {
+    return false;
+  }
+
+  public static boolean isEligible() {
+    return Build.VERSION.SDK_INT < 19;
+  }
+}

src/org/thoughtcrime/securesms/util/Util.java

@@ -395,8 +395,12 @@ public class Util {
   }
 
   public static int getDaysTillBuildExpiry() {
-    int age = (int)TimeUnit.MILLISECONDS.toDays(System.currentTimeMillis() - BuildConfig.BUILD_TIMESTAMP);
-    return 90 - age;
+    if (Build.VERSION.SDK_INT < 19) {
+      return Integer.MAX_VALUE;
+    } else {
+      int age = (int) TimeUnit.MILLISECONDS.toDays(System.currentTimeMillis() - BuildConfig.BUILD_TIMESTAMP);
+      return 90 - age;
+    }
   }
 
   @TargetApi(VERSION_CODES.LOLLIPOP)