Use "ldrtl" and "ldltr" config qualifiers for RTL / LTR resources

- we cannot use "rtl" / "ltr" qualifiers as they can conflict with ISO-639 Alpha-3
codespace which uses 3 letters for identifying a language code (and could use either
"rtl" or "ltr" strings for defining a language in the future).

- we are using instead "ldrtl" for RTL and "ldltr" for LTR resources. Those qualifiers
are defined by more than 3 chars and outside of what is defined into ISO-639. They
are also more understandable as "ld" prefix is for "layoutdirection"

Change-Id: Id43e948103707e09bef63ebd54ac1779dde58e72