SystemUI: Fix uneven volume icon padding in status bar

The current silent and vibrate volume icons in the status bar have
slightly too much padding compared to all of the other status bar icons,
which makes it look out-of-place. Remove the excess insets to make it
more even.

Note that the silent icon still has 0.5 dp of padding on its left and
right sides to make its total size match that of the vibrate icon, which
is 19 dp, while the silent icon is 18 dp.

Change-Id: I7fb9a6e0113f6103eed308ac2d25c392fe8abb73