libsnapshot: Use std::unordered_set in DmSnapCowSizeCalculator.

There is a check here if vector resize fails. In practice, this would
throw bad_alloc or length_error and cause a runtime abort, so the check
is dead code.

To protect against bad chunk_ids we can switch to unordered_set instead.
The original memory concerns for std::set are less applicable since
unordered_set is bucketed.

Bug: 194431534
Test: apply OTA; run vts_libsnapshot_tests
Change-Id: I09c108b700d2f83acf80a9eaa5099b46aedcab89