libdm: Add dm-crypt and dm-default-key support to libdm.

Also, add an Emplace() method to DmTable to make target construction
easier. For example,

    table.AddTarget(std::make_unique<DmTargetLinear>(...));

Becomes:

    table.Emplace<DmTargetLinear>(...);

Bug: 132206403
Test: libdm_test gtest
Change-Id: Iac62c74546ebaa660cb32d6894a019bdac24a305