add prebuilt_sdk_system_api_android_txt module

Added a filegroup module prebuilt_sdk_system_api_android_txt.

Bug: 190635892
Test: TreeHugger
Change-Id: I6e699ba632c086b3373895825f78210e32ffd8f5
Merged-In: I6e699ba632c086b3373895825f78210e32ffd8f5
diff --git a/Android.bp b/Android.bp
index fdfddfb..1d7e084 100644
--- a/Android.bp
+++ b/Android.bp
@@ -143,5 +143,12 @@
     srcs: [
         "*/system/api/*.txt",
         "*/public/api/*.txt",
+    ]
+}
+
+filegroup {
+    name: "prebuilt_sdk_system_api_android_txt",
+    srcs: [
+        "*/system/api/android.txt",
     ],
 }