libsnapshot: Add a helper for waiting for device paths.

Normally, DeviceMapper::CreateDevice() handles this for us. However, it
does not work in first-stage init, because ueventd is not running.
Therefore this patch adds a way for first-stage init to set a callback
to manually regenerate and process uevents.

Additionally, even with ueventd, dm-user misc device creation needs a
WaitForFile() call, since ueventd is asynchronous.

The WaitForDevice() helper in this patch accounts for both of these
scenarios.

Bug: 173476209
Test: device boots into first-stage init after full VABC ota
Change-Id: Ib7a9bfc2a5a5095aa00b358072f9cb1743c19ab2