IME: Fix broken IME when detaching served (but not next served) view

Fixes an issue where if the mServedView gets detached, but a different
view was already set as the mNextServedView, the next served view gets
mistakenly cleared as well.

Fixes: 216548832
Bug: 211105987
Test: atest ImeFocusControllerTest
Change-Id: I3fe42bcb5b08158cbdbf261ba0227075464ffe4c