Add a min density dpi for external displays.

This CL introduces a minimum bound for the external display density which is currently set as 100. This means If the calculated external display density falls under 100 it will be set to 100.

Bug: 425944604
Flag: com.android.server.display.feature.flags.base_density_for_external_displays
Test: manually tested with low dpi display
Change-Id: I71b7d32c6a7bd6e11937e72b865af79105d6ee91