Improve color, contrast, and dimensions to match new spec for download icon

Here we update the values to the correct HCT values according to the specs, as well as the size of the icon

Bug: 267272201
Test: Turn on flag ENABLE_DOWNLOAD_APP_UX_V3 and download an app to see the changes.
Change-Id: I2cc2417f7500c4c08c41056bfeef3c33f4eab174