Add an “is valid” query to ANativeWindow

This will help avoid certain crash (null dereference),
and return an error instead. This is useful for
app developers as java language apps are not
expected to crash when passed bad arguments

This doesn’t fix bug 36065206, but was prompted
by it.

Test: compile / run
Bug: 36065206
Change-Id: I329874821f5cd780eeaacf8945859af5ab790cf4