Fix undefined fontScale issue in Configuration

When using a Configuration object as a delta for use
as an update to an existing Configuration object,
the fontScale property is always defaulted to 1.0, which
is not considered "undefined". That means that fontScale will
always get overridden to 1.0.

This changes the undefined value of fontScale to 0.0, which is
set when the Configuration object is constructed. Thankfully,
the documentation for Configuration states that until
Configuration#setToDefaults() is called, the Configuration is in
an invalid state. That means that apps can not rely on fontScale == 1.0
without calling setToDefaults().

Bug:29924927
Change-Id: I19342c55f7057423f1ca8c5d8dce1dff07617d90