Statically link more libraries

In commit 4bff0d63e4ca621cb88fac5ff5d363edd6ead8de we modified TestApk
so that the payload statically linked most libraries. But then we
added MicrodroidIdleNativeLib which didn't follow the same pattern.

So,
- Move MicrodroidIdleNativeLib under testapk.
- Statically link its libc++.
- Get rid of libiobenchmark which seemed redundant.

I haven't attempted to remove libbase.so and libc++.so from the
benchmark APK; the files are used both in host & VM, and I suspect
doing so would be a net loss.

Bug: 255920361
Test: Manual: Build, check .so files contained in the APKs
Test: atest MicrodroidTests MicrodroidBenchmarkApp
Change-Id: I6620f179536da39bab3a4ebc4f288af333548562
diff --git a/tests/testapk/Android.bp b/tests/testapk/Android.bp
index 8972046..9b05080 100644
--- a/tests/testapk/Android.bp
+++ b/tests/testapk/Android.bp
@@ -55,3 +55,10 @@
     srcs: ["src/native/testlib.cpp"],
     stl: "libc++_static",
 }
+
+cc_library_shared {
+    name: "MicrodroidIdleNativeLib",
+    srcs: ["src/native/idlebinary.cpp"],
+    header_libs: ["vm_payload_headers"],
+    stl: "libc++_static",
+}