[M120 Migration][MM] Remove esplusplayer dependency from the BufferObserver interface.

[M120 Migration][MM] Remove esplusplayer dependency from the BufferObserver interface.

There is a buffer observer for processing media buffering-related callback
from player.
The current structure is that ESPlusPlayerBufferObserverImpl implements
the BufferObserver interface, which has esplusplayer dependency.
The dependency of certain players is removed from BufferObserver
for scalability to allow Capi mmplayer to also use this interface.

Reference:
https://review.tizen.org/gerrit/#/c/292484/
https://review.tizen.org/gerrit/#/c/292740/

Change-Id: Ie7f5297f37d77f17fe02b8b8a97d903746198e75
Signed-off-by: Sun-woo Nam <sunny.nam@samsung.com>