LayoutLib: Fix EditText rendering

Framework has overloaded java.lang.System.arraycopy() for char[]. The
method is not present on the Desktop VMs. This change replaces the calls
to this method by its more general alternative - the one using Objects.

TODO: Make it more configurable and flexible to allow rewrite of any
such methods in the future.

Change-Id: I6823b13e52b1c555eb257d02b79707d84e73236f