Run unshare for app data isolation for instant app

Even MountEmulatedStorage does not run it, isolateAppData should
run it also otherwise it's mounting on root namespace.

Test: atest --instant CookieManagerTest
Test: atest EphemeralTest#testWebViewLoads
Bug: 148966677
Change-Id: I5e2dd8087461efd6b7d566d089c9223f253f3b4f