custom domx: refactor ENHANCED_DOMX and only use DOMX_PATH checks in Android.mk

Change-Id: I597973b9179cab68834a6f8dc9eb45f43c4af735

Conflicts:

	media/libstagefright/OMXCodec.cpp
	media/libstagefright/id3/Android.mk
	media/libstagefright/timedtext/Android.mk
	media/libstagefright/wifi-display/Android.mk