Respect force_mount_namespace in MountEmulatedStorage().

In Ieb75cc3009ed26b7366213409d5fad836f597084, the unshare step was
skipped if no storage is required. But the change failed to take the force
parameter into account

Test: m
Test: angler boots
Test: fugu boots

Bug: 21643067
Change-Id: I52447f02fd25f553628564733fd6bf2523c07f7c