Register RttManager correctly

RttManager is an exception in that the
service name (Context.WIFI_RTT_SERVICE)
it is published under is different from its
binder name (Context.WIFI_RTT_RANGING_SERVICE).

When
`SystemServiceRegistry.registerContextAwareService()`
tries to get the binder of Context.WIFI_RTT_SERVICE,
it won't find anything since there is no binder
published under that name. Thus, the registration
will fail.

Instead, delegate RttManager construction to
WifiRttManager construction.

Bug: 148817663
Test: atest android.net.wifi.rtt.cts.WifiRttTest
Change-Id: I94e505583689f3fd3836dba2f7feb0e7f0ab1378