Changes quick affordance key to string.

Before this change, the key that uniquely identifies each quick
affordance was the class of that key. Since we're about to embark on a
project where we'll have many implementations that share the same class,
this is no longer sufficient.

Previously, I was reluctant to use a string because there was no easy
way to guarantee uniqueness. Now that we see that we can't guarantee
uniqueness through the class typing system, we have to compromise.

Bug: 254858696
Test: unit tests updated and all passing. Manually verified that
affordances work and that clicking on one doesn't trigger the other.

Change-Id: I2d1ec382c04604c8478737720eb92410eec479f8