Unhide LinkProperties, NetworkCapabilities, NetworkRequest APIs

These methods are marked to @UnsupportedAppUsage APIs since
Android Q. But some system apps still need them to set/get
necessary network or request information. Hence, make them to be
public or system APIs.

Bug: 120448492
Test: atest FrameworksNetTests
Change-Id: I95a44daef5615e290b40d0796ca183b88ad8a63f
diff --git a/core/java/android/net/NetworkCapabilities.java b/core/java/android/net/NetworkCapabilities.java
index 0bdfca7..9acb676 100644
--- a/core/java/android/net/NetworkCapabilities.java
+++ b/core/java/android/net/NetworkCapabilities.java
@@ -976,7 +976,7 @@
      * @return The bearer-specific signal strength.
      * @hide
      */
-    @UnsupportedAppUsage
+    @SystemApi
     public int getSignalStrength() {
         return mSignalStrength;
     }