IOMX: do not convert ANWB to gralloc source in emptyBuffer

Bug: 29422020
Bug: 31412859
Change-Id: If48e3e0b6f1af99a459fdc3f6f03744bbf0dc375
(cherry picked from commit 534bb6132a6a664f90b42b3ef81298b42efb3dc2)
