Allow supported camera locks to change after initialization.

A camera backend may not be able to report which locks it supports
until after it is loaded, so query support on demand instead of caching
the value available at construction time.

Change-Id: I56fb1adee980ebc2fa893da3c983a6dce26329c0
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>