tests: handle legacy and new bootstat complete

Regression from commit cb1a8e7f
("Don't retrigger bootstat during userspace reboot.")

sys.boot_completed and either sys.logbootcomplete (legacy) or
sys.bootstat.first_boot_completed set to determine if the display
has gone active for bootstat.  For adb remount test we have no
desire or need to wait for bootstat complete, reduce script
complexity.

Also solve a possible problem with no content supplied to xargs.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Test: adb-remount-test.sh
Bug: 148804390
Bug: 135984674
Change-Id: Ieefddf583ff7422e8811d2e338a0f16c8943b0d7