ui: bring back donation button

For Google Play Store builds, we'll display an alert box. This was
inspired by the discussion around StreetComplete; hopefully we'll have a
similar okay outcome.

Link: https://github.com/streetcomplete/streetcomplete/issues/3768
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

ui/src/main/res/values/strings.xml

@@ -108,6 +108,9 @@
     <string name="tv_select_a_storage_drive">Select a storage drive</string>
     <string name="tv_no_file_picker">Please install a file management utility to browse files</string>
     <string name="tv_add_tunnel_get_started">Add a tunnel to get started</string>
+    <string name="donate_title">♥ Donate to the WireGuard Project</string>
+    <string name="donate_summary">Every contribution helps</string>
+    <string name="donate_google_play_disappointment">Thank you for supporting the WireGuard Project!\n\nUnfortunately, due to Google\'s policies, we\'re not allowed to link to the part of the project webpage where you can make a donation. Hopefully you can figure this out!\n\nThanks again for your contribution.</string>
     <string name="disable_config_export_title">Disable config exporting</string>
     <string name="disable_config_export_description">Disabling config exporting makes private keys less accessible</string>
     <string name="dns_servers">DNS servers</string>

ui/src/main/res/xml/preferences.xml

@@ -40,4 +40,6 @@
         android:summaryOff="@string/allow_remote_control_intents_summary_off"
         android:summaryOn="@string/allow_remote_control_intents_summary_on"
         android:title="@string/allow_remote_control_intents_title" />
+    <com.wireguard.android.preference.DonatePreference
+        android:singleLineTitle="false" />
 </androidx.preference.PreferenceScreen>