Make IME source frame not smaller than nav bar frame

In gesture navigation, nav bar frame is larger than frame to calculate
insets. IME should not provide frame which is smaller than the nav bar
frame. Otherwise, nav bar might be overlapped with the content of the
client when IME is shown.

Fix: 142218188
Test: atest DisplayPolicyTests
Change-Id: Ie5edcd1a0f73440623e85aa4a020d16b4266f747