Set background color as black for wearable web apps

Set background color as black for wearable web apps

[Model] COMMON
[BinType] AP
[Customer]

[Issue#] P170609-03650
[Request] N/A
[Occurrence Version] DayR765AUC_20170607.002

[Problem] White background displayed for KA Stock App instead of
          black background
[Cause & Measure] Cause: Background color is set as black only for
                  watchfaces. Hence default bg color for web apps is
                  white
                  Measure: Call ewk_view_bg_color_set to set background
                  color as black for web apps

[Checking Method] 1. Build and install crosswalk-tizen rpm
                  2. Install KA Stock web app
                  3. Background color of web app should be black

Change-Id: I50f8c24f9f735ebb0ef24f49d9e8a1ebdbb11999
Signed-off-by: Guneet K <g.khosla@samsung.com>