snapuserd: Refactor how buffers are managed.

Currently all I/O is processed through BufferSink. However, the
allocation of buffers is implicit in low-level helper functions, which
have no knowledge of how much data will actually be sent. As a result,
"allocation of buffers" and "tracking of how many bytes were written" is
disjoint. This will make it very difficult to break dependence on
dm-user, without a much more complex API.

This patch refactors how BufferSink is used. First, GetPayloadBuffer has
been deprecated in favor of AcquireBuffer. AcquireBuffer performs both
allocation and write tracking. The number of bytes written is not
necessarily the number of bytes allocated, and the new API allows for
this.

The "Process" helpers now take in an explicit buffer, and their callers
are responsible for allocating a buffer. This allows for
ReadUnalignedSector to have proper buffer offset tracking.

Because write tracking is now accurate, no explicit size needs to be
passed to WriteDmUserPayload. It can simply read the current watermark
in BufferSink.

This patch also removes XorSink, since its dependence on BufferSink made
this change more difficult.

Bug: 288273605
Test: snapuserd_test
Change-Id: Id9ca5044f9c0386f351b250349793b6b276b01b7