Skip IME show on unsecured displays

If IME is attempted to be shown on untrusted display, skip it instead of
throwing exception. Split and re-use the existing
WMS#shouldShowIme(displayId).
Since in new Insets world, IME window show is requested by client
window, INTERNAL_SYSTEM_WINDOW permission check is not required.

Bug: 111084606
Test: atest CtsWindowManagerDeviceTestCases
Change-Id: I3231e93b1d4cb79ef70e22cb620f5aaf50e0f3b1