Add led capability LIGHTS_ADJUSTABLE_BATTERY_LED_BRIGHTNESS

*) Many existing liblights support notification brightness
   control via the alpha channel but do not support similar
   for the battery led.

*) https://github.com/LineageOS/android_lineage-sdk/commit/8f7a4559ab4a81855f399eae32ebe4a3f531e8b2
   introduced a dependency on light capability
   LIGHTS_ADJUSTABLE_BATTERY_LED_BRIGHTNESS such that, when
   set, it was assumed that the battery led brightness is
   also liblights adjustable.  It turned out that this
   was not the case.

*) Create a new capability to allow devices to distinguish
   whether their liblights supports adjustable brightness
   control for notification and battery leds independently
   of one another.

*) Document how brightness support works in LightsCapabilities.

*) Copy LightsCapabilities comments to the default lights config
   to raise visibility.

Change-Id: Id95905f07128c78c6acacf3b60d71bb37f70bcfb