[CD Taskbar] Bypass binder calls for TaskbarManager#getDefaultDisplayId()

Calls to TaskbarManager#getDefaultDisplay() were making binder calls each time, which caused a crash in cases where binder is blocked. Now, I cache the default displayId avoid these calls.

Flag: EXEMPT not adding new behavior
Bug: 400606685
Test: m
Change-Id: I4a0c0d0c99efb137119b2299d3259541379a6bb6