fix(overlay): default to global directionality

[crisbeto]

Currently overlays default to the ltr direction, which means that consumers, whose apps are in RTL, have to pass in the direction explicitly for every overlay. These changes switch to taking the default direction from the global Directionality which is based on the body and html elements.

Fixes #9817.

[jelbourn]

LGTM

[philip-firstorder]

@crisbeto Shouldn't this also re-position the Pasta 3 (Spaghetti) and Pasta 4 (Tortellini) in the overlay-demo.ts when toggling the RTL button in the header?

I just tested now with your commits and the elements don't automatically reposition themselves.

[crisbeto]

@philip-sultanescu this only determines the default direction for overlays, it's still up to the overlays to reposition themselves when the direction changes.

[philip-firstorder]

@crisbeto But shouldn't they automatically reposition themselves like all the other controls do when the bidi event is triggered?
It seems like an overhead to have to program it specifically for the overlays.

[crisbeto]

That's because the OverlayRef doesn't have a reference to the dir instance that is specific to the trigger. We could add a parameter for it, but it seems like overkill for some that doesn't happen often.

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