Make APIs in the base CellInfo class public.

Test: build
Bug: 139417139
Change-Id: I591c500136ac1f28d9cb87cd1649fd891e1315f6

api/current.txt

@@ -44461,6 +44461,8 @@ package android.telephony {
   public abstract class CellInfo implements android.os.Parcelable {
     method public int describeContents();
     method public int getCellConnectionStatus();
+    method @NonNull public abstract android.telephony.CellIdentity getCellIdentity();
+    method @NonNull public abstract android.telephony.CellSignalStrength getCellSignalStrength();
     method public long getTimeStamp();
     method public boolean isRegistered();
     field public static final int CONNECTION_NONE = 0; // 0x0

telephony/java/android/telephony/CellInfo.java

@@ -189,11 +189,15 @@ public abstract class CellInfo implements Parcelable {
         mTimeStamp = ts;
     }
 
-    /** @hide */
+    /**
+     * @return a {@link CellIdentity} instance.
+     */
     @NonNull
     public abstract CellIdentity getCellIdentity();
 
-    /** @hide */
+    /**
+     * @return a {@link CellSignalStrength} instance.
+     */
     @NonNull
     public abstract CellSignalStrength getCellSignalStrength();