Ensure notification status bar icons scale well on low-density displays

On low-density displays, notification icons appeared too small because
their view's ScaleType was being set to `CENTER_INSIDE`. This scale type
does not scale up drawables that are smaller than their container,
resulting in a small icon centered within a larger, square view.

This CL resolves the issue by changing the ScaleType to `FIT_CENTER`.
This ensures the drawable is correctly scaled up to fit its container
bounds while preserving the aspect ratio.

Test: IconManagerTest
Test: Simulate a low density display with `adb shell wm density 88` and
      `adb shell wm size 1080x1920`
Flag: com.android.systemui.shared.status_bar_connected_displays
Bug: 413620149
Change-Id: If00d1aea7937a1031661821ca46080d5bd8a19b2