Improve testDevicePolicyIme

Make sure we're correctly testing devicePolicy enforcement by also testing
restoring IME after policy is removed from editor.

Bug: 139872425
Test: atest CtsInputMethodTestCases
Flag: android.view.inputmethod.enforce_device_policy_ime
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:8ccbf711b617382f95398795cd5d7272eb8437fa
Merged-In: I397c992e95380437c601f14914ff32198289dd14
Change-Id: I397c992e95380437c601f14914ff32198289dd14