Improve EventHub const-correctness

Ensure that all methods that "get" state from event hub are
const-correct. While some "set" methods can also be const in event hub
because they write to the system (side effect) instead of changing
state, we leave these as non-const to make it easier for testing.

Bug: None
Test: atest inputflinger_tests
Change-Id: I086b50458203a2395b1960e2bc1102610a3c0801