Build 64-bit mediaserver on 64-bit-only

This change also updates the mediaresourcemanager tests to be buildable
on both 32-bit and 64-bit, without adding additional libraries to the
system image.

Bug: 16890215
Change-Id: Icf8f1d0b098217d34bf27911a72346124588cc5b
diff --git a/media/mediaserver/Android.bp b/media/mediaserver/Android.bp
index a968890..56b59e2 100644
--- a/media/mediaserver/Android.bp
+++ b/media/mediaserver/Android.bp
@@ -33,8 +33,10 @@
         "frameworks/av/services/mediaresourcemanager",
     ],
 
-    // back to 32-bit, b/126502613
-    compile_multilib: "32",
+    // mediaserver has only been verified on 32-bit, see b/126502613
+    // use "prefer32" to *only* enable 64-bit builds on 64-bit-only lunch
+    // targets, which allows them to reach 'boot_complete'.
+    compile_multilib: "prefer32",
 
     init_rc: ["mediaserver.rc"],