rootdev: Don't try to access /dev/block.

When the device used to back a path is found in /sys/block, rootdev will
double check that /dev/block/<device name> exists and that the dev_t
matches the one in /sys/block/.
On Android, the manufacturer can add an SELinux context for that device
node which will prevent core daemons from accessing it, failing the call
to rootdev.

To avoid this, rootdev should return the device node path without trying
to access it.

BUG: 24143423
BUG: 24267261
TEST: metricsd starts and find the main disk without any SELinux denial.

Change-Id: I4c0fb2800ac84ff451974990c6228ee10528c0f0