feat(menu): coercing overlapTrigger attribute

[ocarreterom]

Uses coerceBooleanProperty for overlapTrigger attribute.

<md-menu overlapTrigger="false"></md-menu>

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[ocarreterom]

I signed it!

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[jelbourn]

coerceBooleanProperty should only be used for things that start as false and are optionally turned to true.

@kara was overlapTrigger meant to be set to true by default?

[kara]

Yes, it should overlap the trigger by default

[jelbourn]

This will be captured in #7283

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