Set initial screen brightness earlier in the boot process.

Previously we had to wait for systemReady before setting the brightness
due to the order in which the display power controller was initialized.
Unfortunately it could take us a rather long time to reach that stage,
particularly after an OTA where the screen would remain at maximum
brightness for minutes while "Optimizing Apps".

This change moves the brightness backlight setting code deeper
into the display manager which has a couple of nice side-benefits
in that it now becomes much easier to coordinate display power mode
changes with display backlight changes.  So this change also resolves
some issued with changing the backlight while in DOZE_SUSPEND and
ensuring that backlight changes generally end up being performed
before executing a power mode change except in the case where the
display needs to come out of suspend first.  (So now the backlight
will be set before entering DOZE from the ON state.)

Deleted some dead code in LightService which was in the way.

Bug: 19029490
Change-Id: I494b5223e676248daf2ff8be3ec338845977f73c