Show mobile signal strength in QS edit panel

Apparently ImageView#setImageDrawable() will take the drawable you give
it and reset its level. Pre-empting that by setting the ImageView level
first fixes that.

Change-Id: I4233241df1e4425ad2c4f5c32599608354a10c30
Fixes: 63690824
Test: visual; runtest -x \
frameworks/base/packages/SystemUI/tests/src/com/android/systemui/qs/SlashImageViewTest.java