update_verifier: Handle legacy care_map.txt gracefully.

update_verifier should be backward compatible to not reject legacy
care_map.txt from old releases, which could otherwise fail to boot into
the new release.

For example, we've changed the care_map format between N and O. An O
update_verifier would fail to work with an N care_map.txt - a) we have
switched update_verifier to read from device mapper in O; b) the last
few blocks that contain metadata can't be read via device mapper. This
could be a result of sideloading an O OTA while the device having a
pending N update.

Bug: 63544345
Test: As follows on sailfish:
 1. Flash the device with this CL;
 2. Put a copy of N care_map.txt at /data/ota_package/. Restore the
    permissions properly ('cache' group);
 3. `adb reboot bootloader`;
 4. `fastboot set_active <current_slot>`
 5. Device boots up into home screen, with a warning in logcat that says
    it has skipped legacy care_map.txt.
Change-Id: I6acc88c9e655a9245e6531f176fef7953953935f
(cherry picked from commit 5a1dee01df3af346729b5791606b72d59b8e9815)