Add default values for InputTarget struct

The struct InputTarget has some default values for its fields, but not
for all fields. Now that we are adding a FocusEntry, we don't want to
deal with scale and such. To prevent undefined values in this struct,
add default initializers.

Bug: none
Test: none
Change-Id: I47660f81a642f55182dd90e91dc19e17f4e2f1dd