Allow IME hiding on displays without system decorations

This CL allows for displays without system decorations to hide the IME
if WM#setDisplayImePolicy(DISPLAY_IME_POLICY_HIDE) is called.

Fixes: 174523386
Test: atest CtsWindowManagerDeviceTestCases:MultiDisplaySystemDecorationTests#testDisplayPolicyImeHideImeNoSystemDecorations
Change-Id: I612800e6787edbff8abe536d33a20146264893d1