Create builtin display tokens on demand

For hotpluggable builtin displays (currently just HDMI), create the
display device IBinder token when the display is connected and destroy
it when the display is disconnected. Previously we created the tokens
at startup and never changed them. This made it so that when comparing
current and drawing state, we couldn't tell whether a display had been
disconnected and reconnected.

Bug: 7491120
Change-Id: I2ac82b864e10cb1cd0a308782d7e0ab9745c5d81