Move system_shared_libs into target.bionic clause

Use target.bionic.system_shared_libs when it is used to limit the
default shared libraries (as opposed to remove them completely).
This avoids attempting to add a host dependency on libc when
system_shared_libs is modified to apply to all variants.

Bug: 193559105
Test: m checkbuild
Change-Id: I5dff12fd48a40a788e78b18c050f806da987754e
diff --git a/Android.bp b/Android.bp
index bbcb86d..045924c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -40,9 +40,10 @@
         darwin: {
             enabled: false,
         },
+        bionic: {
+            system_shared_libs: ["libc", "libdl"],
+        },
     },
-
-    system_shared_libs: ["libc","libdl"],
 }
 
 cc_binary {