sepolicy: unify *_contexts file install location

This change installs *_contexts files to the same location on Treble and
non-Treble devices.

This was previously not possible because first stage mount was not
required on all platforms. It is now b/79758715.

Bug: 70851112
Test: m selinux_policy
Change-Id: I8124c59b129aef86d78d2ae4ebcfaecd896032fc