Trebuchet: Hide Notification Dots on low RAM devices

 * As the feature is not available for low RAM devices,
   hide it from the Settings to avoid confusion

 * Acquire ActivityManager service to read isLowRamDevice

Change-Id: I34a6ee1b4838f31194c89327e301d4fd39670c02
Signed-off-by: Adrian DC <radian.dc@gmail.com>