sm8150-common: Set gain of vibrator to 0x55 on boot

* Seems better than the default of 0x80, vibrator is
   a bit too strong by default.

Change-Id: I676b74aac3cbc1900c73b3236879479e8ecfca00

rootdir/etc/init.target.rc

@@ -203,6 +203,10 @@ on boot
     restorecon /sys/kernel/debug/regmap/0-0042/registers
     restorecon /sys/kernel/debug/regmap/tavil-slim-pgd/registers
 
+    # Reduce default vibrator gain from 0x80 to 0x55,
+    # also allow gain customization via persistent property
+    write /sys/class/leds/vibrator/gain ${persist.vendor.vib.gain:-0x55}
+
 on property:vendor.display.lcd_density=560
    setprop dalvik.vm.heapgrowthlimit 256m