Revert "ANDROID: Incremental fs: Avoid continually recalculating hashes"

This reverts commit ab185e45.

This change used the PageChecked flag to mark the Merkle tree as
checked. However, f2fs uses this internally. This caused file system
hangs on devices after installs.

Test: incfs_test passes, installs no longer hang
Bug: 157589629
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I980a700d65eb4f4a77434715d61dda4b8e80658c