ANDROID: dm: rename dm-linear methods for dm-android-verity

This keeps linear_target as static variable and just exposes
the linear target methods for android-verity

Cherry-picked: https://android-review.googlesource.com/#/c/212858



Change-Id: I4a377e417b00afd9ecccdb3e605fea31a7df112e
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
(cherry picked from commit a6d1b091f40b25d97849487e29ec097bc5f568dd)