libsnapshot: Refactor COW reading/writing of footers.

This simplifies the footer and label resume system for COW files.
Previously, a footer could be missing, and we'd try to recover the file.
Any ops we could recover were handed off to the CowWriter, which then
searched up to the correct resume label.

In the new model, a reader is opened with an optional label (resume
mode), or without a label.

When resuming to a label, a footer is optional. The read is terminated
by EOF, discovery of the requested label, or the presence of a footer.
However the requested label must be found, and parsing fails if it is
not found.

When opening a COW without a label, the footer must be present, as this
signals the file was successfully flushed. Parsing fails if no footer is
found.

Bug: N/A
Test: cow_api_test
      full OTA with VABC
Change-Id: Ie79ab5259f532dd16c72f0e42da7568c5c5c4623