Make FUSED_PROVIDER SystemApi

This should always have been SystemApi, as most fused providers are
implemented outside system server. There is a strong argument this
should actually be public API, but I'm punting on that for now until we
can give it more consideration.

Bug: 173030969
Test: none
Change-Id: If5ba10accbe2f6ce47d749536d053e1bd8e297f8

api/system-current.txt

@@ -4190,6 +4190,7 @@ package android.location {
     method @RequiresPermission(android.Manifest.permission.WRITE_SECURE_SETTINGS) public void setLocationEnabledForUser(boolean, @NonNull android.os.UserHandle);
     method @Deprecated @RequiresPermission(android.Manifest.permission.WRITE_SECURE_SETTINGS) public boolean setProviderEnabledForUser(@NonNull String, boolean, @NonNull android.os.UserHandle);
     method @Deprecated @RequiresPermission(android.Manifest.permission.LOCATION_HARDWARE) public boolean unregisterGnssBatchedLocationCallback(@NonNull android.location.BatchedLocationCallback);
+    field public static final String FUSED_PROVIDER = "fused";
   }
   public final class LocationRequest implements android.os.Parcelable {

core/api/system-current.txt

@@ -4130,6 +4130,7 @@ package android.location {
     method @RequiresPermission(android.Manifest.permission.WRITE_SECURE_SETTINGS) public void setLocationEnabledForUser(boolean, @NonNull android.os.UserHandle);
     method @Deprecated @RequiresPermission(android.Manifest.permission.WRITE_SECURE_SETTINGS) public boolean setProviderEnabledForUser(@NonNull String, boolean, @NonNull android.os.UserHandle);
     method @Deprecated @RequiresPermission(android.Manifest.permission.LOCATION_HARDWARE) public boolean unregisterGnssBatchedLocationCallback(@NonNull android.location.BatchedLocationCallback);
+    field public static final String FUSED_PROVIDER = "fused";
   }
   public final class LocationRequest implements android.os.Parcelable {

location/java/android/location/LocationManager.java

@@ -202,6 +202,7 @@ public class LocationManager {
      *
      * @hide
      */
+    @SystemApi
     @TestApi
     public static final String FUSED_PROVIDER = "fused";