Support of multiple device partitions by dm-user driver

Add basic support for daemon creation and handle signals

Test: Add test case to test system and product COW partitions
Bug: 162790322

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Icf8dbe49d2237cec6f7dbcdd84256eb6c5afa1cd