Expose last access update timestamp.

This commit addresses the first part of [Bug 329021273822477](https://developers.facebook.com/bugs/329021273822477) by introducing a new set of accessors to `com.facebook.android.Facebook`:
* `getLastAccessUpdate()` - retrieves the last time an access token was set/extended
* `setLastAccessUpdate()` - set the last update time for an access token

This prevents `setAccessToken()` from blindly updating `mLastAccessUpdate` when called at application startup to restore a saved session, thus restoring `extendAccessTokenIfNeeded()` functionality.

Bundled examples have been updated to reflect the use case for the new methods.