Extend adoptable override to force on or off.

Virtual disks are adoptable by default, but for debugging purposes
we want to treat them as unadoptable in some cases.  Add the ability
for the "sm" shell command to force on/off, or return to default.

Bug: 77849654, 74132243
Test: manual
Change-Id: Ieda317396624ca081e5dd9568795483f684f9297