add option to build LPA support in addition to legacy OMX

LPA works on msm7x27a devices which uses ION.
However, legacy OMX stuff is still needed on these targets.

Change-Id: I3e104c005a6522ce314a96045930935aa322ea7f

Conflicts:
	media/libmedia/Android.mk
	media/libstagefright/Android.mk