libsnapshot:snapuserd: Handle un-aligned IO request

If the requested IO is not 4k aligned and spans
between two COW Operations, then we will have
to split the IO as we need to read the partial
buffers from two COW operations.

BUG: 176918488
Test: cow_snapuserd_test - Data verification with unaligned IO
      Full OTA on cuttlefish
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Icf6801e1767112b92cb7991808860f119adebda2