Not trying to remove admins already being removed

If someone calls removeActiveAdminLocked more than once, it is possible
for the device policy data to end up with more than one copy of an admin
in the list mRemovingAdmins. Due to extra entries, once the admin
component is removed, it is not being allowed to be set as an admin again,
until the device reboots or mRemovingAdmins is cleared from the memory
due to some other reason. Fixing this by making sure we do not add
duplicate entries to mRemovingAdmins

Bug: 30369197
Change-Id: I1d53c41312171425bbd6e6e4153148276f1b098d