Remove VisibilityState flags and simplify DefaultImeVisibilityApplier

Previously (before [1]), those flags were used as an indicator whether
the IME should been shown or hidden, As we're not depending on
additional attributes anymore, we can simplify the cases and just call
show/hide directly.

[1]: I8e3a74ee579f085cb582040fdba725e7a63d6b85

Fixes: 433457669
Test: atest DefaultImeVisibilityApplierTest ImeVisibilityStateComputerTest
Flag: EXEMPT refactor
Change-Id: I1399ec6c9e3f5ed70f02ba2326edd0c73eb930b4