Fix another bug with RTL tabs

super.instantiateItem should continue being called with the
original unadjusted position (since getItem already adjusts it for
RTL).

Instead, override getItemId to return the RTL adjusted position,
so that if a fragment is being retrieved from the FragmentManager
instead of being created by getItem, the overriden getItemId will
ensure that FragmentPagerAdapter will construct the correct tag to
retrieve the correct fragment from the FragmentManager.

Bug: 17673059
Change-Id: I42a7ce3f0d8a17384fdb16c998e49254d3a734db