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/src/com/android/browser/BrowserSettings.java b/src/com/android/browser/BrowserSettings.java
index e3d1ee5..864315a 100644
--- a/src/com/android/browser/BrowserSettings.java
+++ b/src/com/android/browser/BrowserSettings.java
@@ -774,6 +774,14 @@
         mPrefs.edit().putBoolean(PREF_AUTOFILL_ENABLED, value).apply();
     }
 
+    public boolean isPowerSaveModeEnabled() {
+        return mPrefs.getBoolean(PREF_POWERSAVE_ENABLED, false);
+    }
+
+    public void setPowerSaveModeEnabled(boolean value) {
+        mPrefs.edit().putBoolean(PREF_POWERSAVE_ENABLED, value).apply();
+    }
+
     // -----------------------------
     // getter/setters for debug_preferences.xml
     // -----------------------------