libsnapshot: Add a debug prop to cause snapshot creation to fail.

This adds a new debug property that causes snapshot creation to fail if
any COW space is needed. This allows us to test recovery fallback code
without generating a huge OTA.

Bug: 416748567
Test: vts_libsnapshot_test
Change-Id: Ia26020b3f9c5e7cf01d22089340203ec99a62b61