libgui: Add dequeue/attach timeout

Adds the ability to specify the timeout when dequeueBuffer or
attachBuffer block due to the lack of a free buffer/slot. By default,
these will block indefinitely (which is signified by a timeout of -1).

When a timeout (other than -1) is specified, non-blocking mode is
disabled and the given timeout will be used instead.

Bug: 25196773
Change-Id: I17fdbeebccb7c8d878703d758ac1209608258e61