Move MetadataHeader to libui

To remove BufferHubMetadata off pdx dependency.

Note that the __attribute__(packed) is removed as part of this CL, as
it's not really needed and is triggering clang warnings.

Test: build passed. Not test needed as no behavior changes.
Bug: 118893702
Change-Id: Ifae94a143a2bedef68a653c57f089b95d166e6d7