Add unit tests and refactor LE legacy pairing key distribution methods

* Add unit test for confirm (c1), p1, and p2
* Refactor the code to reduce number of methods
* Refactor c1's code to make it testable

Bug: 32413756
Test: Code compilation, Unit Test, PTS SM Tests
Change-Id: I5cc876b7dd2b21a0780ac9a5236420223df6cddb