Allow font scaling preference persist. Fixes issue 3530.

This allows fotScale preference set via SpareParts persist. Previous commit (0833f4f7) intruduced a bug resulting in extra-ultra-unreadable-tiny text if user never set a font scale preference before, thus it was reverted by commit Ifaea1156c0ddceaf74974ee387bb19a8cee69a88. Now this should work OK, as Settings provider doesn't allow fontScale to be set to 0. I did a full wipe to confirm the previous bug and did the same to ensure that it's OK now.

Change-Id: I562f80149f9af33674860c285d96ed7ced044422