snapuserd: remove implicit assumption about dm-user in BufferSink

This removes implicit assumption that BufferSink can only be used with
dm-user. User of BufferSink can specify size of header during
BufferSink::Initialize to reserve the space for the header.

Also get rid of now unused structure dm_user_payload and
dm_user_message.

Test: TH
Test: ota
Test: vts_snapused_test
Bug: 357711761
Change-Id: I1e550393ce1f0277fe215015fa6948ad3c3b68ba
Signed-off-by: Sandeep Dhavale <dhavale@google.com>