Allow reading wallpaper with MANAGE_EXTERNAL_STORAGE

We keep the legacy StorageManager check since some CTS still rely on it.

Flag: EXEMPT small easy-to-revert fix
Bug: 341974583
Test: treehugger
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ca66d291162a794df1a5b10176c9e557161fbc1f)
Merged-In: Ife0f4c957694d25e8f5230ac2eb11244a70bb866
Change-Id: Ife0f4c957694d25e8f5230ac2eb11244a70bb866