Add CreateEmptyDevice and WaitForDevice APIs

These APIs support a flow in which dm devices can be created before they
are actually needed, hence minimizing the time a process will wait for
ueventd to create user space paths.

Bug: 190618831
Test: atest libdm_test
Change-Id: I4dfa14e5271a6a13de6da73ec3c7efb1ebc0f8b8