Make OP_NO_ISOLATED_STORAGE accessible via public API

If OP_NO_ISOLATED_STORAGE app-op is granted it gives apps
legacy-app's access. Update public API for apps to check the
state of legacy external storage access isExternalStorageLegacy()
to respect OP_NO_ISOLATED_STORAGE appop.

Bug: 172934418
Test: android.scopedstorage.cts.device.ScopedStorageDeviceTest#testDefaultNoIsolateStorageFlag
Test: atest CtsScopedStorageHostTest
Change-Id: I5cc2dfabedac57fe4183a586549c979145abcbfb