[MM] Use the timer to decide operations according to buffering status.

[MM] Use the timer to decide operations according to buffering status.

Previously, PerformOperationForData was called in ReadBuffer which reads
a buffer from the demuxer. However, ReadBuffer may not be called depending on
the buffering status.
When actually determining an operation, it judges based on the current position
so there are cases that it does not work as intended.
Therefore, call the function periodically using a timer.

This fixes the patch below.
https://review.tizen.org/gerrit/#/c/305478/

Change-Id: I5c4ee665d52b02c609bef8a380d1aa05e5dfb577
Signed-off-by: Sun-woo Nam <sunny.nam@samsung.com>