libsnapshot: add compatibility check

Ensure that cow was written by v3 writer for the data_length check to
work. All ops written by v2 writer should go through the decompressor
path if a compressor was used.

Test: cow_api_test
(cherry picked from https://android-review.googlesource.com/q/commit:04ca59d6ffb1d9f071df1d89a3fdd947f7dbd576)
Merged-In: I053d6fdaf29ef7001e68f43b45d5a3ff1a36b1c3
Change-Id: I053d6fdaf29ef7001e68f43b45d5a3ff1a36b1c3