Disable "internal" when adoption not possible.

When adoption isn't possible, we shortcut to try formatting as
external, but that still results in a warning dialog that the user
could cancel away from and then manually pick "internal."

Bug: 110863258
Test: atest com.android.settings.ui.StorageWizardTest
Change-Id: I04645c3fce11a883c10996772d73440a0dcd51ec