Add direct report back when registerForNtnSignalStrength is invoked

There is a case that caller has requested to report ntn signal strength, but no report for a whille as there is not signal strength happens.
Therefore added a immediate report back when registerForNtnSignalStrength is invoked

Bug: 353593633
Flag: EXEMPT bugfix
Test: atest SatelliteControllerTest
    manually e2e test and check whether direct reports triggered by log checking
Change-Id: I17bd7f7989171ec64a6b38a73665552a2619080a