tpm2: move policy building out of policy session creation

This retains the use of policy sessions instead of trial sessions
in most cases, based on the code comment that some TPMs do not
implement trial sessions correctly. However, it's likely that the
issue was not the TPMs, but our code's incorrect use of PolicyPCR
inside a trial session; we are not providing expected PCR values
with our call to PolicyPCR inside a trial session, but the spec
indicates that in a trial session, the TPM *may* return error if
the expected PCR value(s) are not provided. That may have been the
source of the original confusion about trial sessions.

More details:
https://github.com/systemd/systemd/pull/26357#pullrequestreview-1409983694

Also, future commits will replace the use of trial sessions with
policy calculations, which avoids the problem entirely.

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