Pre-create userdata metadata encryption device.

CreateDevice() implicitly calls WaitForDevice(), which can impact boot
time if there are many uevents waiting to be processed. To alleviate
this, create an empty "userdata" device when vold starts (if metada
encryption is enabled). When it comes time to actually enable metadata
encryption, the device can be re-used and the subsequent Wait should be
much faster.

Bug: 198405417
Test: manual test; device boots
Change-Id: Iaacd10858272f17353475e25075ea1dda13f8fc4