Adjust IMapperMetadataTypes.h to match gralloc4 encoding

Specifically include the header encoding. Initially
omitted as it's a bit redundant, the value in having
bit-for-bit identical encoding between gralloc4 &
imapper5 seems worthwhile enough to keep it.

Test: impltests + VtsHalGraphicsMapperStableC
Change-Id: Iee37bb97acf40362c301a06f9118938b1a0c2cd9