libsnapshot: Remove GetSnapshotSize

A single number is not enough to represent the ranges
that needs to be snapshotted in the target partition. For
prototyping now, just use device_size instead.

Add test in PartitionCowCreatorTest to ensure that this
doesn't regress. Also fix some trivial tests.

In follow-up CLs, snapshot ranges should be represented
with a more complicated structure. See b/141889746.

Bug: 141889746
Test: libsnapshot_test

Change-Id: I1a508c2464abce216ad4049cc2533ffdaa8cd14f