ClientModeImpl: Don't enable RSSI polling on secondary CMM

Since none of the current seconday CMM use-cases can become the
default network when cellular is present, turn off rssi polling (which
gates a bunch of interactions like scoring, sending usability stats
to external scorer, etc).

Bug: 181693252
Test: atest com.android.server.wifi
Change-Id: I026ed5125b32ebc62e1da871f0e565575dc7cda9