proguard: Also keep FixedScaleDrawable

FixedScaleDrawable is used for legacy icons if LEGACY_ICON_TREATMENT is
set to false. However, it gets removed by proguard, this patch adds a
keep rule.

Change-Id: Ifcedad251013abefbb52cd7bc0ed2f59aa1bc06f
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>