Default dual tone alpha set to 0.3

In the event where a user of ThemedBatteryDrawable does not set the
foreground / background colors explicitly, but rather uses a color
filter, just set a default alpha for the background paint.

Test: manual; set theme to any dualtone icon pack and visit settings
battery page
Bug: 162901879

Change-Id: If9becd40fbebeba15f3714e012623d3da61a7efe