[SB] Fix status bar icon when a layer-list contains some level-lists.

If an app sends a status bar icon that is a layer-list containing 1 PNG
layer and 1 level-list layer:

```
<!-- status_bar_icon.xml -->
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/close_png"/>
    <item android:drawable="@drawable/level_example"/>
</layer-list>

<!-- level_example.xml -->
<level-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:maxLevel="3333" android:drawable="@drawable/ic_example_low" />
    <item android:maxLevel="6666" android:drawable="@drawable/ic_example_medium" />
    <item android:maxLevel="10000" android:drawable="@drawable/ic_example_full" />
</level-list>
```

Then SysUI was incorrectly showing the icon with level=0 always, even if
the app requests a different level via
`.setSmallIcon(R.drawable.status_bar_icon, /* level= */ 7000`.

This is because `DrawableSize` was scaling down the icon, and during
that process it seems SysUI lost the information about which drawable to
use for different levels.

This CL updates `DrawableSize` to not scale down icons if the icon is a
LayerDrawable whose layers are different drawable types.

Fixes: 244282477
Flag: EXEMPT bugfix

Test: run test app that posts a notification using status_bar_icon.xml
with level=4000, verify status bar icon (and AOD icon and shelf icon)
show ic_example_medium in the icon. With level=7000, verify
ic_example_full is in the icon
Test: verify all other notification icons work as before
Test: atest DrawableSizeTest

Change-Id: I9b6afd0bc223b4bde597cf5ec42078054a6b81fe