display: plugin: Add display_plugin_get_device_flags()

display: plugin: Add display_plugin_get_device_flags()

The reason why the display_plugin_get_device_flags() is added is as follows
1. device flags was being used in the lcd_on/off_procedure() through get_lcd_on/off_flags()
2. There is no need for the function to be divded to get_lcd_on/off_flags()
3. Also, ambient flags is only used in the wearable plugin.

Thus, display_plugin_get_device_flgas is a combination of get_lcd_on/off_flags().
It provides device flags value to get_lcd_on/off_flags().
However wearable uses ambient flags, so wearable gets own get_device_flags().

Change-Id: Ibcf4825b7028f045b721e6bfe4e429b77746fe69
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>