add java_api_finder plugin to mainline module DocumentsUI to generate java APIs used by DocumentsUI.

Bug: 129294170
Test: m DocumentsUI RUN_ERROR_PRONE=true |& tee documentsui_error_prone.out
Change-Id: I44f2e6657a0e036267ec6563891a2aa95f4cf0f9
Merged-In: I44f2e6657a0e036267ec6563891a2aa95f4cf0f9
(cherry picked from commit 9afb4f9a5895481e1ba2a71c91798bb3bb785f4a)
diff --git a/Android.bp b/Android.bp
index 764dc49..a05de4d 100644
--- a/Android.bp
+++ b/Android.bp
@@ -39,6 +39,8 @@
     sdk_version: "system_current",
     min_sdk_version: "28",
     target_sdk_version: "28",
+
+    plugins: ["java_api_finder"],
 }
 
 filegroup {