3 uninitialized variables in IOMX.cpp

Uninitialized MetadataBufferType pointer in case:
SET_INPUT_SURFACE
STORE_META_DATA_IN_BUFFERS
CREATE_INPUT_SURFACE

Fix: initialize them to kMetadataBufferTypeInvalid

Bug: 26324358
Change-Id: Ib3bcac9bedc98bc65efa39f67cdbffbd90b374c9
