fs_mgr: overlayfs test fix adb_sh

Some commands like 'adb_sh grep " rw," /proc/mounts' do not preserve
the embedded spaces when transferred to the device and thus do not
entirely behave as expected.  There are several commands issued this
way throughout the test that are similarly broken.  To add
consistency, pass any arguments that are not explicitly (double)
quoted and contain embedded spaces as quoted to preserve.

Also devices that report the kernel internal device /dev/root in
/proc/mounts should be suppressed when investigating whether
overlayfs is needed.  If /dev/root is reported, then we should assume
that the mount point / represents it for the following df command
when trying to determine if the partition is right-sized.

When there is only one device attached, and a serial number is not
specified, correctly determine this condition to progress to
the testing.

Test: adb_remount_test.sh
Bug: 122231184
Change-Id: I913e297dd272d6691110bb5d7a7c7f54321b6833