Request MANAGE_EXTERNAL_STORAGE permission

This allows com.android.storagemanager to actually manager external
storage, e.g. clean up files contributed by other apps.

This would auto-grant All Files Access to com.android.storagemanager on
devices where the app is preinstalled, but it can still be revoked from
the special app access page.

Test: build + verify app has M_E_S
Test: atest GtsStorageTestCases
Fix: 160550958
Change-Id: I91a9ba173b27fe05474b69c5e5e2fb860b8debb6