[Mobile] Show the roaming icon in the top-right on RTL.

The old roaming drawable took up the full space of the signal icon, so
it couldn't be moved around using gravities alone. Reducing its size
to be just the size of the R lets us move it around correctly in RTL.

Fixes: 230795372
Test: `adb shell am broadcast -a com.android.systemui.demo -e command
network -e mobile show -e level 2 -e datatype roam -e slot 4 -e roam
show` -> see roaming icon in top-left of signal icon.
Test: above command in RTL -> see roaming icon in top-right of signal
icon.

Change-Id: Ie8be70a3c92e88bcfe0a9bdd9610995ccbfd9529