libsnapshot: refactor CreateUpdateSnapshots

This function is too big. Refactor it a bit.

- Reorder checks / variable initializations a bit.
  They are independent of each other, so they can be
  safely reordered.
- Put snapshot creation and initialization to their own
  functions so that CreateUpdateSnapshots looks shorter.

Test: libsnapshot_test
Change-Id: I135b415d8e046dd91d31326fe7962ae44be4ccf8