init: Reorder GetBlockDeviceSymlinks() so FindDmDevice() is first

By moving FindDmDevice() it's easier to unify the code with the
IsBootDevice() function. In this case the order doesn't matter since
anything with the uevent path "/devices/virtual/block/dm-" (the only
devices FindDmDevice() looks at) won't match any of the other sections
of the if/then/else test.

Bug: 316324155
Test: Build & boot

Change-Id: I819eb60aa5077f0eb2c2f2783b152e43a52ba8b7