Use Math.floor to convert RSSNR from ten db to DB

To keep the same pictogram level with Modem, use Math.floor
to convert RSSNR for LTE.

Bug: 196280737
Test: atest SignalStrengthControllerTest
Change-Id: I40797ba39ccd8d7bd802e912f8005ee9f580977d

telephony/java/android/telephony/CellSignalStrengthLte.java

@@ -600,7 +600,7 @@ public final class CellSignalStrengthLte extends CellSignalStrength implements P
 
     /** @hide */
     public static int convertRssnrUnitFromTenDbToDB(int rssnr) {
-        return rssnr / 10;
+        return (int) Math.floor((float) rssnr / 10);
     }
 
     /** @hide */