system_info_generated: Rearrange <enum> be a child of <key>

system_info_generated: Rearrange <enum> be a child of <key>

Previously, the <key> and <enum> are at the same level. Changed it so
that the <key> now adopts an <enum> as a child only when its @type="enum".
The implementation follows the concept that the <enum> is subordinate
to the <key>. That is, the operations of the class 'TizenFeatureEnum'
are controlled by its parent, the class 'TizenFeature'.
  - Parsing 'TizenFeature' is followed by parsing 'TizenFeatureEnum'
  - Printing 'TizenFeature' is followed by printing 'TizenFeatureEnum'

Change-Id: I683c84823f67fce91c8f5703a037f4bf7c24adbf
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>