ANDROID: Incremental fs: Avoid continually recalculating hashes

With a verified file (use incfs_perf to create a verified file), throughput
measured using dd after dropping caches increases from 200M/s to 290M/s

Test: incfs_test passes
Bug: 155996534
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I7abb5ad92e4167f82f3452acc9db322fec8307dd