Initialise the brightness value with Float.NaN in PersistentDataStore

If there is no DisplayState, and it gets initialised because of any
other field, for instance refresh-rate, the brightness gets initialised
by 0. Since 0 is a permissible value for brightness, is causes issues by
changing the brightness to 0.

Bug: 259517441
Test: atest PersistentDataStoreTest
Change-Id: I532afc9e9d437199922f83f3c4440127a86a88f5