Add a power saving mode setting

Create a setting that tunes performance optimizations and improves the
power usage.
Automatically tune mode with the Power-saving mode of Android Lollipop+.
Also, when the phone drops below threshold for low battery, prompt user
to turn browser's power save mode on.

Change-Id: I64089a371de562f9a18c297eb09555733f020515
diff --git a/res/xml/general_preferences.xml b/res/xml/general_preferences.xml
index a1f2d25..d0bf533 100644
--- a/res/xml/general_preferences.xml
+++ b/res/xml/general_preferences.xml
@@ -37,6 +37,12 @@
             android:title="@string/pref_lab_fullscreen" />
 
         <CheckBoxPreference
+            android:defaultValue="false"
+            android:key="powersave_enabled"
+            android:summary="@string/pref_powersave_enabled_summary"
+            android:title="@string/pref_powersave_enabled" />
+
+        <CheckBoxPreference
             android:key="autofill_enabled"
             android:title="@string/pref_autofill_enabled"
             android:summary="@string/pref_autofill_enabled_summary"