SystemUI: Refactor statusbar bluetooth icon

Move it to its own layout like wifi and mobile,
and separate the bluetooth connected and battery
icons.

This allows us to use bluetooth connected icon
from icon packs, and also color the battery icon
when its at low battery (in the next change).

While we're at it, simplify the logic used to
determine drawable from battery level, and also
add icon for <10% battery.

Change-Id: I5dd50d84497816484e1c33a7432a3d85c3a7f79d
Co-authored-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>