Don't allow legacy apps to create things in /storage/emulated.

This dir may be writable to MediaProvider on devices without
sdcardfs, but we don't actually want to allow apps to write to it.

Bug: 157530951
Test: atest
android.appsecurity.cts.ExternalStorageHostTest#testExternalStorageWrite
  atest CtsScopedStorageHostTest

Change-Id: Ie9f58110e189cbfc37a4944d6300aca87b43402d
1 file changed