DataSource: Remove global variables in DataSource related to sniff

Removed global variables related to sniff, and added Sniffer class
to implement the sniff functionality with non-static members.

Change-Id: I6fbd0ba5b686e09fda11d78cdd687a69c81a6bdf
CRs-Fixed: 567753

Conflicts:
	include/media/stagefright/DataSource.h
	media/libstagefright/DataSource.cpp

Change-Id: I9751bf75dc7d0fbfbb5f734f66e86aa52a4238e6