Don't alter accessibility JS APIs unless a page is about to load.

Previously we were adding and removing the APIs on window attach and
detach, but a page's life cycle is not related to attach/detach. This
patch also ensures that ChromeVox is not null before calling. Includes
formatting fixes and extra comments in the waitForResultTimedLocked
method to improve readability. Fixes parsing of integer resultId, which
was being parsed using Long.

Bug: 7328348
Change-Id: I6b81a8e4d8209f8e99419da9b8250abe57e25048