Expose GSM Timing Advance, Update LTE TA docstring

-Expose the GSM timing advance
-Add a spec reference to the LTE Timing Advance

Bug: 28910982
Test: None
Change-Id: I7573dbb1316a2ab52051072ebad2532601f4299c

api/current.txt

@@ -36855,6 +36855,7 @@ package android.telephony {
     method public int getAsuLevel();
     method public int getDbm();
     method public int getLevel();
+    method public int getTimingAdvance();
     method public int hashCode();
     method public void writeToParcel(android.os.Parcel, int);
     field public static final android.os.Parcelable.Creator<android.telephony.CellSignalStrengthGsm> CREATOR;

api/system-current.txt

@@ -39790,6 +39790,7 @@ package android.telephony {
     method public int getAsuLevel();
     method public int getDbm();
     method public int getLevel();
+    method public int getTimingAdvance();
     method public int hashCode();
     method public void writeToParcel(android.os.Parcel, int);
     field public static final android.os.Parcelable.Creator<android.telephony.CellSignalStrengthGsm> CREATOR;

api/test-current.txt

@@ -36932,6 +36932,7 @@ package android.telephony {
     method public int getAsuLevel();
     method public int getDbm();
     method public int getLevel();
+    method public int getTimingAdvance();
     method public int hashCode();
     method public void writeToParcel(android.os.Parcel, int);
     field public static final android.os.Parcelable.Creator<android.telephony.CellSignalStrengthGsm> CREATOR;

telephony/java/android/telephony/CellSignalStrengthGsm.java

@@ -140,6 +140,16 @@ public final class CellSignalStrengthGsm extends CellSignalStrength implements P
         return level;
     }
 
+    /**
+     * Get the GSM timing advance between 0..219 symbols (normally 0..63).
+     * Integer.MAX_VALUE is reported when there is no RR connection.
+     * Refer to 3GPP 45.010 Sec 5.8
+     * @return the current GSM timing advance, if available.
+     */
+    public int getTimingAdvance() {
+        return mTimingAdvance;
+    }
+
     /**
      * Get the signal strength as dBm
      */

telephony/java/android/telephony/CellSignalStrengthLte.java

@@ -206,8 +206,10 @@ public final class CellSignalStrengthLte extends CellSignalStrength implements P
     }
 
     /**
-     * Get the timing advance value for LTE.
-     * See 3GPP xxxx
+     * Get the timing advance value for LTE, as a value between 0..63.
+     * Integer.MAX_VALUE is reported when there is no active RRC
+     * connection. Refer to 3GPP 36.213 Sec 4.2.3
+     * @return the LTE timing advance, if available.
      */
     public int getTimingAdvance() {
         return mTimingAdvance;