Introduce preserveLegacyExternalStorage manifest attribute

This attribute will default to false and does nothing in the false
state.

In the true state, it will allow an updating app preserve any legacy
storage access from the existing installed version on device.

Test: atest RestrictedPermissionsTest
Test: atest RestrictedStoragePermissionSharedUidTest
Bug: 148944140
Change-Id: Id5b6d7aadd9d9844c8bd1164087a9e093481e234

api/current.txt

@@ -1082,6 +1082,7 @@ package android {
     field public static final int preferenceScreenStyle = 16842891; // 0x101008b
     field public static final int preferenceStyle = 16842894; // 0x101008e
     field public static final int presentationTheme = 16843712; // 0x10103c0
+    field public static final int preserveLegacyExternalStorage = 16844312; // 0x1010618
     field public static final int previewImage = 16843482; // 0x10102da
     field public static final int primaryContentAlpha = 16844114; // 0x1010552
     field public static final int priority = 16842780; // 0x101001c

core/res/res/values/attrs_manifest.xml

@@ -1770,6 +1770,21 @@
              -->
         <attr name="requestLegacyExternalStorage" format="boolean" />
 
+        <!-- If {@code true} this app would like to preserve the legacy storage
+             model from a previously installed version. Note that this may not always be
+             respected due to policy or backwards compatibility reasons.
+
+             <p>This has no effect on the first install of an app on a device.
+             For an updating app, setting this to {@code true} will preserve the legacy behaviour
+             configured by the {@code requestLegacyExternalStorage} flag. If on an update, this
+             flag is set to {@code false} then the legacy access is not preserved, such an app can
+             only have legacy access with the {@code requestLegacyExternalStorage} flag.
+             <p>
+
+             The default value is {@code false}.
+             -->
+        <attr name="preserveLegacyExternalStorage" format="boolean" />
+
         <!-- If {@code true} this app declares that it should be visible to all other apps on
              device, regardless of what they declare via the {@code queries} tags in their
              manifest.

core/res/res/values/public.xml

@@ -3018,6 +3018,7 @@
       <!-- @hide @SystemApi -->
       <public name="isAutofillInlineSuggestionTheme" />
       <public name="allowNativeHeapPointerTagging" />
+      <public name="preserveLegacyExternalStorage" />
     </public-group>
 
     <public-group type="drawable" first-id="0x010800b5">