adbd: Fix adb remount when switching roots during first-stage init.

This patch addresses two issues with adb remount.

First, the blk_device field in fstab_rec will be incorrect if referring
to a device-mapper node (fs_mgr_update_logical_partition must be
called).

Second, devices that are not system-as-root now move their mount point
and chroot to "/" during first-stage init. However, the fstab entry will
have "/system" as its mount point. The new logic now reflects this.

Bug: N/A
Test: adb remount on hikey or device with dynamic partitions
Change-Id: I48b379db49d2def555ea49888c4994fbcf501672