Move compatibility files out of prebuilts dir.

The treble compatibility tests check for policy differences between old
and new policy.  To do this correctly, we must not modify the policy which
represents the older policies.  Move the files meant to be changed to a
different location from the ones that are not meant to be touched to avoid
any undesired changes to old policy, e.g. commit:
2bdefd65078d890889672938c6f0d2accdd25bc5

Bug: 36899958
Test: Build-time tests build.
Change-Id: I8fa3947cfae756f37556fb34e1654382e2e48372