[Android] Honor prefs.json

[paulrouget]

#20912 followup

Use:

• shell.searchpage instead of the hardcoded reference to duckduckgo in MainActivity.java
• shell.homepage as a default value for mInitialUri in ServoView.java
• honor shell.keep_screen_on.enabled and shell.native-orientation

Snippet from the previous Android port:

        if (BuildConfig.FLAVOR.contains("vr")) {
            // Force fullscreen mode and keep screen on for VR experiences.
            keepScreenOn = true;
            mFullScreen = true;
        }
        else {
            keepScreenOn = preferences.optBoolean("shell.keep_screen_on.enabled", false);
            mFullScreen = !preferences.optBoolean("shell.native-titlebar.enabled", false);

            String orientation = preferences.optString("shell.native-orientation", "both");

            // Handle orientation preference
            if (orientation.equalsIgnoreCase("portrait")) {
                setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
            }
            else if (orientation.equalsIgnoreCase("landscape")) {
                setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
            }
        }