remount: Auto remount option (-R) also tries to gain root

When doing an auto remount & reboot command and is running as the SHELL
uid, just try to gain root on behalf of the user and retry the remount
command.
If "gain root" failed, then print the message to tell the user to run
"adb root" and retry.

Bug: 322285923
Test: adb unroot && adb remount -vR
Test: adb unroot && adb shell remount -vR
Change-Id: If8e04dc602573c73178c108ef4944f0a985b590e