Persist screen brightness setting through upgrade

The screen brightness float setting initially did not exist when upgrading the device software. This change ensures the float and int values are synchronised on the system start-up.

Bug: 167651957
Test: manual - set autobrightness off, upgrade from Q to R, and check brightness value in settings.
Change-Id: I2a3b996c8747e3c5f1d181bbdd438c70bf23d08b
(cherry picked from commit 96f43f21b3a3021762c2d213d8958590127cae36)