Don't lose product variant strings that also vary between locales.

Localized strings with product variants were not being included in the APK,
apparently because the check to ensure that a different variation of the
string had not already been included in the APK was matching the version
of it from the default, untranslated configuration.

Now check to make sure that the string not only exists but also exists in
the correct configuration.

Bug 5372711

Change-Id: I52975570b75e0f11827dc6bcf1cb4a987d0541aa