Let IME be aware of display override configuration updates

This patch contains three parts:
- Register a listener to corresponding ActivityDisplay to let IME get
  updated display override configuration.
- Isolate IME display override configuration update from general global
  default display configuration update.
- Fix a possible NPE in getProcessGlobalConfiguratoin().

With the patch, IME will keep to get correct process global
configuration of the display it's registered to.

Bug: 113253755
Bug: 116870598
Test: See repro steps in b/111364446#comment10.
Test: go/wm-smoke
Change-Id: I7fb58ed23e58e4696a2fe336806c4bf323891074