create_snapshot: Derive source hash from device

Verity hash extracted from device merkel tree is stored
in a file. Use that to derive the source hash

Bug: 371304627
Test: create_snapshot --source=system.pb --target=$OUT/system.img --merkel_tree
Change-Id: I670318496054d5aa9f81efa5954e4fdd28bc3e19
Signed-off-by: Akilesh Kailash <akailash@google.com>