Stop supporting legacy ConnectivityManager routing methods in M.

The methods startUsingNetworkFeature, stopUsingNetworkFeature and
requestRouteToHost were @removed in all the M preview builds, but
internal and external developers have noted that this imposes
additional burden for applications that need to work across
multiple platform versions because it causes compile-time errors.

We switched from @removed back to @deprecated to avoid these
problems. In order to effectively deprecate these methods, which
are error-prone and insecure, make them throw
UnsupportedOperationException if the app's target SDK is M or
above.

Because there are still one or two places in system code that use
these APIs, exempt Process.SYSTEM_UID and the OMA-DM client from
the check for now.

Bug: 22728205
Change-Id: I790bd32f3aa8067cbb625962a209bb9232f4b58c