Load preinstalled sytem font if fs-verity is not supported

Some form factor doesn't support fs-verity and it ends up with NPE
during boot because of empty system font map.
To work on such environment, load preinstalled system font before
serializing system font map.

Bug: N/A
Test: N/A
Change-Id: Ibc4d512e1a70d9e7be889ea7b1e6afa5e45b1bbb