The correct constant is KeyCodes.KEYCODE_MEDIA_PLAY_PAUSE
It seems to depend on the Bluetooth Audio Driver, which Constant is used.
On my HTC Keycodes.KEYCODE_MEDIA_PLAY_PAUSE works fine, but
on newer devices (ACER A500 and Galaxy S2)
Keycodes.KEYCODE_MEDIA_PLAY and
Keycodes.KEYCODE_MEDIA_PAUSE
are sent in the Activity_Keypress Event instead of
Keycode.KEYCODE_MEDIA_PLAY_PAUSE
from Bluetooth Audio.
All three Constants are listed in the Android Documentation.