feat: support lazy-loading HammerJS w/ Angular 6.1

Angular 6.1 adds the HAMMER_LOADER token that can be used to lazy-load
HammerJS. This commit updates Angular Material's gesture config to be
compatible with lazy-loading HammerJS by:
* Only reading `window.Hammer` after Angular knows that HammerJS has
been loaded.
* Always registering the set of gesture events used in the Angular
Material components (instead of registering no events when Hammer is
absent).

Once we are able to depend on HAMMER_LOADER itself (in v7.0), this can
be simplified a bit.

Author: jelbourn