fs_mgr: differentiate if fs_mgr_set_verity() was skipped or disabled

In case of non-secure builds (eng variant) fs_mgr_setup_verity() skips
verity checks regardless of fstab options. This is slightly different
than 'adb disable-verity' where it would first read the verity metadata
to check if verity is disabled.

So, this change adds a new return value of FS_MGR_SETUP_VERITY_SKIPPED
instead of piggy backing on the FS_MGR_SETUP_VERITY_DISABLED.

Bug: 62864413
Test: Boot sailfish

Change-Id: I42bf2bdce0ecb18b4c3b568e2bc96bf1590dfb35
Signed-off-by: Sandeep Patil <sspatil@google.com>