Move calculating base density for external displays to LogicalDisplay.

This is needed for the upcoming synthetic mode resolution change that will override the resolution in LogicalDisplay.
Having density calculation in LogicalDisplay solves the density update issue on resolution change.

Bug: 438485020
Test: atest LogicalDisplayTest
Flag: com.android.server.display.feature.flags.base_density_for_external_displays
Change-Id: I799332ecfb5ef9f83e239d75e1ba82d0a2bd73f7