Stop disabling the UnavailableSymbol check

Doesn't appear to be necessary.

Bug: 306806877
Test: m checkapi
Change-Id: I9458dc58ba46b7111937d096545618c7166ec33c
diff --git a/Android.bp b/Android.bp
index 0c08119..03fdc29 100644
--- a/Android.bp
+++ b/Android.bp
@@ -691,7 +691,6 @@
         "--hide RequiresPermission",
         "--hide SdkConstant",
         "--hide Todo",
-        "--hide UnavailableSymbol",
         "--hide-package android.audio.policy.configuration.V7_0",
         "--hide-package com.android.server",
         "--manifest $(location :frameworks-base-core-AndroidManifest.xml)",