freeze compat matrix 6 for Android S

The current matrix is left in this same CL because this way, downstream
modifications to the current CL will automatically get absorbed into the
level 7 compatibility matrix. A CL on top of this will disable the 7
matrix and then get reverted in the aosp/master..goog/master merge
path.

Bug: 178221726
Test: boot device, vts_treble_vintf_vendor_test
Test: inspect matrix by diffing matrix with old matrix using:
    m analyze_matrix &&
    system/libvintf/analyze_matrix/hals_for_release.py
Change-Id: Id83986fc5089eefc2292f0042753f739f4e01a44