ANDROID: dm: use default verity public key

If the dm-android-verity target does not provide a default
key try using the default public key from the system keyring.
The defualt verity keyid is passed as a kernel command line
argument veritykeyid=.

The order of the dm-android-verity params have been reversed
to facilitate the change.

Old format example:
dm="system none ro,0 1 android-verity Android:#7e4333f9bba00adfe0ede979e28ed1920492b40f /dev/mmcblk0p43"

New formats supported:
dm="system none ro,0 1 android-verity /dev/mmcblk0p43 Android:#7e4333f9bba00adfe0ede979e28ed1920492b40f"

(or)

dm="system none ro,0 1 android-verity /dev/mmcblk0p43"
when veritykeyid= is set in the kernel command line.

BUG: 28384658
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: I506c89b053d835ab579e703eef2bc1f8487250de
(cherry picked from commit c5c74d03)