libdm: Add DmTargetTypeInfo and a unit test.

This change refactors DmTarget. It was satisfying two separate use cases
that have no overlap: (1) as a container for informational queries, and
(2) for specifying table targets.

The kernel's nomenclature for the former is a "target type," so the new
class is named DmTargetTypeInfo.

In addition, this change adds a unit test for
DeviceMapper::GetAvailableTargets that ensures the device has at least a
linear target type (with more TBD).

Bug: 110035986
Test: libdm_test gtest
Change-Id: Icb87976801e8a1e6ec79e48b1d58c308d36279e5