Create sdk build target for AppSearch.

Bug: 228981572
Test: Presubmit
Change-Id: Ie2827c07df8ec378b2957a74fd2b915bf468fa6b
diff --git a/Android.bp b/Android.bp
index 5ff276c..09d235c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -31,6 +31,10 @@
     ],
 }
 
+sdk {
+    name: "appsearch-sdk",
+    bootclasspath_fragments: ["com.android.appsearch-bootclasspath-fragment"],
+}
 
 apex {
     name: "com.android.appsearch",