snapuserd: Refactor code to a seperate directory

Move all the code relevant to snapuserd to a seperate
directory. Add OWNERS file.

No other code changes apart from moving files around
and fixing couple location of header paths
at few places.

Bug: 194642092
Test: Compile, Full OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ib1d852bfeda4eca5c996d6cd7b057f141cb5ddad