Rename InputWindowHandle hasFocus to focusable

A window with hasFocus set to true, means the window can be focusable.
The current name is confusing when mutliple windows set the field to
true. Rename this to focusable and also remove canRecieveKeys field which
is not used.

Test: presubmit
Test: drag freeform windows

Bug: 151179149
Change-Id: I1b2354da0dc241d5e887e599e43f0426671289b8