Check updatable font dir in FontListParser.

If updatable font dir is provided by caller and fonts exist in the dir,
FontListParser will use the updated fonts.
Currently, updatable font dir is used only in test.

Bug: 173619554
Test: atest TypefaceSystemFallbackTest
Change-Id: I631764fe235d258b6b5cb6d6906c17d48c7e4608