Add not exported flag to affected runtime receiver

Android T allows apps to declare a runtime receiver as not exported
by invoking registerReceiver with a new RECEIVER_NOT_EXPORTED flag;
receivers registered with this flag will only receive broadcasts from
the platform and the app itself. However to ensure developers can
properly protect their receivers, all apps targeting a future
platform release and registering a receiver for non-system
broadcasts must specify either the exported or not exported flag
when invoking #registerReceiver; if one of these flags is not
provided, the platform will throw a SecurityException. This commit
updates the receiver in RespondViaSmsManager with the
RECEIVER_NOT_EXPORTED flag since this receiver is receiving
broadcasts sent via PendingIntents sent to the SmsManager.

Bug: 234659204
Test: Build
Change-Id: I603cab48e1e0098a218a24d0e2a4b6c2669ce7f2