GestureNav: Fix bug in exlcusion limit device config

Fixes an issue, where the DeviceConfig exclusion limit was reset
whenever another key in the android:window_manager namespace changed.

Fixes: 140190037
Test: Set limit device config flag to value >200, set unrelated flag, verify limit is still >200.
Change-Id: I2df5b6661cc5e545d7c3de38dec5c86e66baea22
Merged-In: I2df5b6661cc5e545d7c3de38dec5c86e66baea22