InputManagerService: Reload pointer icons only when density changes

Reloading pointer icons is a non-trivial task that requires I/O
for reading icons from files. Reduce the number of times icons need to
be reloaded by only reloading icons when the display density changes.
Other display changes, such as orientation changes, do not need the
pointer icons to be reloaded.

Bug: 321324470
Bug: 325133368
Test: adb shell wm density X
Change-Id: I9e502f3ba054da0b74f8006f9efcd53f3552b181