Expose prefetching strategies to services

Make the prefetching strategies public so a service can choose
which strategy works best in a particular spot. This should reduce
unnecessary/redundant prefetching.

For example, the FW currently only allows hybrid descendant
prefetching, but a service may want to do depth-first or
breadth-first traversal of the view hierarchy.

Currently, if there is another user interactive request, we
immediately return prefetched nodes. Also allow services to prevent
this interruption and force prefetching to a max of 50 nodes.

Services could potentially request a certain number of nodes, but
since asynchronous prefetching immediately returns the requested
node, the  service can force prefetching of 50 nodes if desired,
and only exposing strategies touches less code, I prefer limiting this.

Also use a LinkedHashMap so ordering is kept when prefetching
descendants.

Test: Manual, talkback builds
atest AccessibilityCacheTest,
AccessibilityInteractionControllerNodeRequestsTest
Bug: 192489177

Change-Id: I3d8358411ece5d2e1380282824cd3cf1835658ac