Merge "libdm: Add a helper function for creating placeholder devices." am: 3460b752

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2357843



Change-Id: I99d9fa79092b0980136236aebc47b743bdf4675a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>