Always calculate default property values

We used to only calculate the default property value if the attribute
was not set in the XML. The properties palette can make use of the
default property values even if the value is manually set so adding code
to support that use case.

Bug: http://b.android.com/210957
Change-Id: Iaade37cbe78329f7be3d34ffbf8fc8e7449cfcaa
(cherry picked from commit 1a6c26f66fd355822a0fa7e08c9ca93582e9fc8d)