Re-implement mixed provider (fix #145)

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

@@ -21,6 +21,7 @@
     <string-array name="settings_provider_names">
         <item>GPS provider</item>
         <item>Network provider</item>
+        <item>Mixed provider</item>
     </string-array>
     <string name="settings_foreground_title">Foreground service</string>
     <string name="settings_foreground_summary">Increase service priority</string>

app/src/main/res/values/values.xml

@@ -4,6 +4,7 @@
     <string-array name="settings_provider_values" translatable="false">
         <item>gps</item>
         <item>network</item>
+        <item>mixed</item>
     </string-array>
 
 </resources>