Add taskbarFrameHeight getters to SystemBarUtils

Add shared utils to get taskbarFrameHeight accessible from both core and
shell. This is required to predict desktop windowing mode insets when
current display is using gesture navbar outside of desktop windowing.

Flag: EXEMPT adding getters
Test: None
Bug: 280496681
Change-Id: Id4104280a3a085b8c67382e6802b4d1b97723c9a