fastbootd: Wait for /dev/block paths when opening logical partitions.

Note that in addition to waiting for the path to appear, we must also
wait for it to be unlinked. Otherwise, we could accidentally access an
older device when opening and closing the same partition twice in a row.

Bug: 114198005
Test: fastboot flashall works
Change-Id: Iddffc34e1ac8aa066c28e7b1a92b09b6dfd7945c