Allow compos and virt APEXes to use private APIs

They are non-updatable APEXes (yet!), thus using private APIs is fine.
Currently, the only private APIs required are in libbinder_rpc_unstable.

Bug: 191637950
Test: m com.android.compos com.android.virt
Merged-In: I4761fc59c15a1925d0df0c3d89081c05dbe875dd
Change-Id: I4761fc59c15a1925d0df0c3d89081c05dbe875dd
(cherry picked from commit 373b032073c4ffe98a80c45e7aa00c6e881da4c8)
diff --git a/apex/Android.bp b/apex/Android.bp
index bf38860..1b0abf4 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -7,6 +7,7 @@
 
     // TODO(jiyong): make it updatable
     updatable: false,
+    platform_apis: true,
 
     manifest: "manifest.json",