[Tizen][Runtime] Fix allow-navigation element can not take effect if there are more than 1 in config.xml.

In Tizen core spec, it's said that "if more than 1, the first occurrence MUST be considered (and all others
ignored)." This patch makes the runtime following the spec.

BUG=XWALK-1696
Merge pull request #2143 from takethathe/fix-duplicate-element

[Tizen][Runtime] Fix allow-navigation element can not take effect if there are more than 1 tags in config.xml