[flexiglass] Verify that NSSL#mImeInset is not used

We needed the ImeInset to scroll to an activated RemoteInput, but it is
handled by the placeholder. This Cl verifies that it is not accessed,
and stops setting it with flexiglass.

Bug: 360091533
Test: run sysui with, and witout flexiglass
Flag: EXEMPT mechanical refactor
Change-Id: Icc598e0750393c7997a59fd1a2394b2a4bc8c4a2