Promote testing framework to a proper platform backend

... which helps to enable building a Gfxstream VK ICD which runs
the host server in-process and can be used for testing with
regular apps (so things outside of the End2End tests). This also
helps to minimize the amount of the things that the End2End tests
are configuring.

Bug: b/293897204
Test: GfxstreamEnd2EndTests
Change-Id: I148d2703744f4bd3e6bf5f111377134abfb5e618
diff --git a/host/Android.bp b/host/Android.bp
index 17fdda2..eda1b6e 100644
--- a/host/Android.bp
+++ b/host/Android.bp
@@ -19,6 +19,7 @@
 cc_library_shared {
     name: "libgfxstream_backend",
     defaults: [ "gfxstream_defaults" ],
+    vendor_available: true,
     cflags: [
         "-Wno-unused-parameter",
         "-Wno-unused-variable",
@@ -104,7 +105,7 @@
         android: {
             srcs: ["NativeSubWindow_android.cpp",],
         }
-    }
+    },
 }
 
 cc_test_library {