Always poll signal strength after setting SignalStrengthUpdateRequest

Justification to apply this planned behavior in S release:

-Cost. In worst case, the SS update request has already waken up modem
 if it was idle. An additional SS polling is almost cost free.
-Benefit. All clients (if present) either are unsolicitedly observing
 SS change or proactively get SS later will receive refreshed SS value.
-Correctness. SS update request and polling are performed within a
 single message handling which is data synchronization boundary to make
 sure shared states are all refreshed when the polling occurs.
-Robustness. The time window between SS update request sent from clients
 and the next SS value received from system is closed. This eliminates
 the need to handle those edge cases occur during the time window.
-Risk. The change has minor risk and the evaluation above shows it is
 very worthy to deploy to S release.

Bug: 178415786
Test: atest com.android.internal.telephony.ServiceStateTrackerTest
Change-Id: Ie96bc2865f4015c3d16c5d2f313fa3064838ef9d