Test CannotCreateEarlyBootKeys only if metadata encryption is on

In R, earlyBootEnded is called only when metadata encryption is being
set up, and thus is not called if metadata encryption is not enabled.
So don't test it under those circumstances.

Bug: 170875742
Test: Cherry-pick aosp/1515419 so that this test can fail. Test passes
    with metadata encryption enabled or disabled, but does not pass
    if metadata encryption is enabled and earlyBootEnded is
    commented out. (Cuttlefish)
Change-Id: I4f1704ca6235d47a1b47902ebbcc1720d016e8dd