hterm: Move default preferences to a property.
hterm: Move default preferences to a property.

* Moved the definition of the default preferences to a property on
  hterm.PreferenceManager rather than an object literal in the constructor.
  This lets embedders adjust default values before the terminal is initialized.

Change-Id: I76e4b3c5213f2547ca7bf94e22dcb8521fcb3b39
Reviewed-on: https://chromium-review.googlesource.com/209820
Reviewed-by: Edoardo Spadolini <9148f5ee85453be54648fd4055bd734c78c2bbb3@gmail.com>
Tested-by: Edoardo Spadolini <9148f5ee85453be54648fd4055bd734c78c2bbb3@gmail.com>
Reviewed-by: Robert Ginda <4419ac4e35d191308e944202d542b192188656d1@chromium.org>

hernad/libapps,hernad/libapps,hernad/libapps,hernad/libapps