Public APIs for ID-specific Uris, misc items.

We have getContentUri() for entire collections of items, but we
only have ID-specific overloads for some of the MediaStore classes;
let's get them all added for consistency.

Remove primary/secondary directory logic, which was replaced by
new RELATIVE_PATH column before Q launched.

Bug: 137890034
Test: atest --test-mapping packages/providers/MediaProvider
Exempt-From-Owner-Approval: trivial API refactoring
Change-Id: Iae4e7fe57adff071c35af459e31223a1fd05fef2
diff --git a/core/java/android/os/Environment.java b/core/java/android/os/Environment.java
index 3462d1f..6d5fe53b 100644
--- a/core/java/android/os/Environment.java
+++ b/core/java/android/os/Environment.java
@@ -168,8 +168,11 @@
         return DIR_ANDROID_ROOT;
     }
 
-    /** {@hide} */
-    @TestApi
+    /**
+     * Return root directory where all external storage devices will be mounted.
+     * For example, {@link #getExternalStorageDirectory()} will appear under
+     * this location.
+     */
     public static @NonNull File getStorageDirectory() {
         return DIR_ANDROID_STORAGE;
     }