Freeze compat matrix 7 for Android T

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 8 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: 203103997
Test: boot, vts_treble_vintf_vendor_test
Change-Id: I5c04d26e2b4dd0aac6e7d6ce3c72496081e9539d