Clear PersistentSystemFontConfig before update.

Currently config.updatedFontDirs is not cleared, so old entries will
accumulate in XML when fonts are updated multiple times.

Bug: 184975511
Test: atest FrameworksServicesTests:UpdatableFontDirTest
Change-Id: Ic4e0ed7d05b55280600ae07fdb190f35484bd307