libstagefright: unbreak hardware OMX on OMAP3

http://review.cyanogenmod.org/58070 has mistakenly reverted
http://review.cyanogenmod.org/44365 which broke hardware OMX
on OMAP3.

The fix for OMAP3 was just a logic change to prevent an
indefinate loop since the nIndex value was being overriden by
the OMX component. The code was modified to use a seprate control
value other than nIndex itself and doesnt pose any issues with
other platforms.

Change-Id: I8ca4f9d48c3f9241f21c9fe6ab98d0be5783ceab
(cherry picked from commit ef3711cc964b0ee9d6af001b49a8ebfbaaf611ab)