Add missing default dirs to MediaProvider

MediaProvider creates the default directories when a new volume it
attached, and also ensures that no app can create a top level directory
if it's not a default directory. This means that any default directory
that MediaProvider doesn't know about cannot be created (by apps that
abide by scoped storage, at least). This change adds 2 missing
directories:
    * Audiobooks
    * Documents

Test: build
Bug: 150695960
Merged-In: I5b07176eb3821933f3f1d13345c5f642bb6dc8b7
Change-Id: I82224b016881fba74f34481ae0bf1f6c36f7059f
diff --git a/src/com/android/providers/media/MediaProvider.java b/src/com/android/providers/media/MediaProvider.java
index d21a060..2b79131 100644
--- a/src/com/android/providers/media/MediaProvider.java
+++ b/src/com/android/providers/media/MediaProvider.java
@@ -574,6 +574,8 @@
             Environment.DIRECTORY_MOVIES,
             Environment.DIRECTORY_DOWNLOADS,
             Environment.DIRECTORY_DCIM,
+            Environment.DIRECTORY_AUDIOBOOKS,
+            Environment.DIRECTORY_DOCUMENTS,
     };
 
     private static boolean isDefaultDirectoryName(@Nullable String dirName) {