[Tizen] Add unit test for metadata configuration element handler

This patch adds tizen_metadata_handler_unittest.cc. Now test verify
that the handler properly handles the following configuration elements:
widget.metadata @key @value
Speed up GetDefaultWidgetEntryPage function

This patch introduces three improvements in the function looking
for widget startup page. There were unnecessary tests when the
result was already found.
'WidgetHandlerTest' test case extension

Add validation for manifest and widget preferences for:
WidgetHandlerTest.ParseManifestWithAllOfOtherItemsAndOnePreferenceItem
test case.
Merge pull request #2727 from aszafranek/WidgetHandlerTestExtension

'WidgetHandlerTest' test case extension
Merge pull request #2703 from KFilipek/AddMetadataUnitTest

[Tizen] Add unit test for metadata configuration element handler
Merge pull request #2744 from tweglarski/GetDefaultWidgetEntryPageSpeedUp

Speed up GetDefaultWidgetEntryPage function