Expose clearCapabilities for mainline module

Support telephony mainline module usage.

Bug: 138306002
Test: Build and make api-stubs-docs-update-current-api
Change-Id: I0a462da4e87cae2c7d4a9b9a92de061a086c5752

api/current.txt

@@ -29477,6 +29477,7 @@ package android.net {
     method public android.net.NetworkRequest.Builder addCapability(int);
     method public android.net.NetworkRequest.Builder addTransportType(int);
     method public android.net.NetworkRequest build();
+    method @NonNull public android.net.NetworkRequest.Builder clearCapabilities();
     method public android.net.NetworkRequest.Builder removeCapability(int);
     method public android.net.NetworkRequest.Builder removeTransportType(int);
     method public android.net.NetworkRequest.Builder setNetworkSpecifier(String);

core/java/android/net/NetworkRequest.java

@@ -247,9 +247,8 @@ public class NetworkRequest implements Parcelable {
          * removing even the capabilities that are set by default when the object is constructed.
          *
          * @return The builder to facilitate chaining.
-         * @hide
          */
-        @UnsupportedAppUsage
+        @NonNull
         public Builder clearCapabilities() {
             mNetworkCapabilities.clearAll();
             return this;