Detect interrupted update due to power off

An interrupted update may stash extra blocks in /cache, leading to a
failure when checking the cache size. We can save the incremented
retry_count in the BCB before installing the update; and distinguish
a fresh update from an interrupted one this way.

Bug: 68679601
Test: An interrupted update reapplies successfully.
Change-Id: Ic1403e1fd25a937c91ef34c14b92a0f6c8f1c0f4