Slices feature supports firmware version.

- Migrate FirmwareVersionPreferenceController to BasePreferenceController class.
- Because slices feature needs an unique uri,
  we should avoid same firmware key in my_device_info.xml and device_info_settings.xml.
- Modify test case for new firmware version controller.

Test: slice uri > content://android.settings.slices/intent/my_device_firmware_version
Test: make RunSettingsRoboTests -j
      atest UniquePreferenceTest SettingsGatewayTest
Change-Id: If516bbdb3ddb823db9909123ef694a2beaf5dc66