libgui: Set HDR metadataSize data type as uint32_t

sizeof(metadataSize) is different in 32-bit or 64-bit mode if data type
of metadataSize is size_t. Fix metadataSize in uint32_t for flatten and
unflatten.  In addition, we take hdr10plus array element size into
consideration when allocate memoery for flatten and unflatten.

b/127370432
Test: check size of metadataSize in flatten and unflatten in hdr10plus
playback.
Test: Check hdr10plus array content when array element size is
non-uint8_t such as uint32_t.

Change-Id: Id7a415887b43c440e3f23ea2f6b201d45271c160