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: 174508435
Test: manual - set autobrightness off, upgrade from Q to R, and check
brightness value in settings.

Change-Id: I2a3b996c8747e3c5f1d181bbdd438c70bf23d08b
Merged-In: I2a3b996c8747e3c5f1d181bbdd438c70bf23d08b
(cherry picked from commit 96f43f21b3a3021762c2d213d8958590127cae36)