gui: add persistent notification and run NfdService in foreground to improve stability

Change-Id: I47e7ff07888e4638a75ddb607d335511467a7779
refs: #4766
diff --git a/app/src/main/res/drawable-hdpi/nfd_notification.png b/app/src/main/res/drawable-hdpi/nfd_notification.png
new file mode 100644
index 0000000..a93def4
--- /dev/null
+++ b/app/src/main/res/drawable-hdpi/nfd_notification.png
Binary files differ
diff --git a/app/src/main/res/drawable-mdpi/nfd_notification.png b/app/src/main/res/drawable-mdpi/nfd_notification.png
new file mode 100644
index 0000000..b51fea8
--- /dev/null
+++ b/app/src/main/res/drawable-mdpi/nfd_notification.png
Binary files differ
diff --git a/app/src/main/res/drawable-xhdpi/nfd_notification.png b/app/src/main/res/drawable-xhdpi/nfd_notification.png
new file mode 100644
index 0000000..588f755
--- /dev/null
+++ b/app/src/main/res/drawable-xhdpi/nfd_notification.png
Binary files differ
diff --git a/app/src/main/res/drawable-xxhdpi/nfd_notification.png b/app/src/main/res/drawable-xxhdpi/nfd_notification.png
new file mode 100644
index 0000000..a099777
--- /dev/null
+++ b/app/src/main/res/drawable-xxhdpi/nfd_notification.png
Binary files differ
diff --git a/app/src/main/res/values/colors.xml b/app/src/main/res/values/colors.xml
index b9519a1..c5320ab 100644
--- a/app/src/main/res/values/colors.xml
+++ b/app/src/main/res/values/colors.xml
@@ -2,4 +2,5 @@
 <resources>
     <item name="ndn_color_fire_bush" type="color">#DB9710</item>
     <item name="android_color_gray" type="color">#C6C6C6</item>
+    <item name="notification_color_orange" type="color">#CD5F31</item>
 </resources>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 47032af..eb7b812 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -114,4 +114,8 @@
     <string name="fragment_route_details_next_hops">List of next hops</string>
     <string name="fragment_route_route_name_title">Route Name</string>
     <string name="ping_client_prefix_hint">Prefix (e.g., /ndn/edu/arizona)</string>
+
+    <string name="notification_content_title">Status</string>
+    <string name="notification_content_text">NFD is running</string>
+    <string name="channel_description">NDN Android notification.</string>
 </resources>