Fix installation of widgets with defaultlocale

Fix installation of widgets with defaultlocale

[Issue#]   WGL-560, WGL-563
[Problem]  Installation of test widgets (with defaultlocale attribute in
configuration file) fails
[Cause] Installer application does not have access to WRT I18N DB and
backendlib does not attach to WRT I18N DB
[Solution] Restore LanguageSubtagRstTreeSingleton used for checking
language tag validity

[Verification]
1. Build repository
2. Run test widgets from WGL-560
(https://bugs.tizendev.org/jira/browse/WGL-560)

[SCMRequest] Depends on: https://review.tizendev.org/gerrit/92271

Change-Id: I3c8d97e62c5f292b00b71874fe73a56aaedbea44