Disable updatable system manifest for T

Bug: 195732042

Merged-In: I6b890e5536c17dd53722e2c3a781c85dd9d6bdf0
Note: the merged-in id is not related with this change but is something
that was already merged in internal master and we don't want this change
to be merged there.

Test: pre-submit + check auto-merger comment in the CL

Change-Id: Idcff9b310ee3e7238e7fd4982d083d496fda9d9e