ANDROID: Incremental fs: Build merkle tree when enabling verity

For incfs files that were created without a merkle tree, enabling verity
requires building a merkle tree first. Although this is the same logic
as verity performs, it is not that easy to reconcile the two given that
incfs has the merkle tree potentially when verity is not enabled.

Bug: 160634504
Test: incfs_test passes
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Ifd304bdad897ff817ce332c62e929a369c2dacdb