Start storageproxyd earlier, but after /dev/sg1 has arrived.

Background:
* Sometimes /dev/sg1 isn't ready when storageproxy attempts to start.
  Add a trigger to wait for the device.

Bug: 245194332
Bug: 245724566
Test: manual
Change-Id: I23300b7f1a0170dc1b73c5e3b5b2c7e2b1ecba2b