Add test to check read/write access in an app's external files dir

This test checks that another app does not have read/write access in an
app's external files directory.

Bug: 156340137
Test: atest android.scopedstorage.cts.host.ScopedStorageHostTest#testReadWriteFilesInOtherAppExternalDir
Change-Id: Iaec2a2c36e2b5e0647b9f5ce9ead044bff11a8b4