Add Policy manager repo

BUG=b/73012579
TEST=Performed repo sync src/third_party/policymanager to see if the
repo gets initialized

Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/21350
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Rayan Dasoriya <dasoriya@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/21810
Reviewed-by: Ke Wu <mikewu@google.com>
Change-Id: I7e1506dc15160795cb7dcee5b0becb461a3a3d69
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/21771