Stylus: Introduce Show Stylus Hover Pointer Setting (2/2)

Currently, the stylus hover icon can only be shown when
config_enableStylusPointerIcon is set to true. This means that the
users do not have a choice of turning the feature on or off.

This CL introduces the capability to do the same. This CL adds the
toggle in the StylusDevicesController to be shown when stylus is
connected. The toggle is mapped to the STYLUS_POINTER_ENABLED secure
setting which InputManagerService listens to turn on and off the stylus
pointer icon.

Test: manual
Steps:
1. Change the value of config_enableStylusPointerIcon as true.
2. Build the images and flash it to a stylus supporting device.
3. After the device is booted, hover over the screen, the stylus
   pointer should appear.
4. Go to Settings > Connected Devices > Stylus and tap on
   "Show pointer while hovering" toggle.
5. Try hovering over the screen, check no pointer icon is shown
6. Repeat Step#4, check that stylus pointer appears.

Run
- make RunSettingsRoboTests \
      ROBOTEST_FILTER=StylusDevicesControllerTest
- atest SettingsProviderTest

Change-Id: Idfff9b4307370cc510a5f94bd57777c30c96e854