Wait for dm device to be ready before format

It can sometimes take a moment for the dm-device to appear after
creation, causing operations on it such as formatting to fail.
Ensure the device exists before create_crypto_blk_dev returns.

Test: adb sm set-virtual-disk true and format as adoptable.
Bug: 117586466
Change-Id: Id8f571b551f50fc759e78d917e4ac3080e926722
Merged-In: Id8f571b551f50fc759e78d917e4ac3080e926722
3 files changed