recovery: Porting screensave mode for new platform.

On new board platform the brightness path of
sys/class/leds/lcd-backlight is deprecated,instead of
/sys/class/backlight/panel0-backlight/.

Test: reboot into recovery on sdm845.
Change-Id: Idf0027ab888f9f982a8eef7de230ce3635e7c300
Signed-off-by: katao <katao@xiaomi.com>