Add setting for configuring zoom level on double-tap.

Added a seek bar in Accessibility settings to configure the zoom level
on double-tap. The range is 75-125% with the default of 100% equal to
the display density.

Bug: 5312461
Change-Id: Iaf1b9f6f6659146db4031c5f819ef9cb55695b5c
diff --git a/src/com/android/browser/PreferenceKeys.java b/src/com/android/browser/PreferenceKeys.java
index 89fcbfc..15ccfe5 100644
--- a/src/com/android/browser/PreferenceKeys.java
+++ b/src/com/android/browser/PreferenceKeys.java
@@ -27,6 +27,7 @@
     static final String PREF_MIN_FONT_SIZE = "min_font_size";
     static final String PREF_TEXT_SIZE = "text_size";
     static final String PREF_TEXT_ZOOM = "text_zoom";
+    static final String PREF_DOUBLE_TAP_ZOOM = "double_tap_zoom";
     static final String PREF_FORCE_USERSCALABLE = "force_userscalable";
     static final String PREF_INVERTED = "inverted";
     static final String PREF_INVERTED_CONTRAST = "inverted_contrast";