[a11y] Move callers of GetNodeFromTree to GetNode

As prep for removing `AXTreeManager::GetNodeFromTree`, this CL migrates
the current callers of GetNodeFromTree to use GetNode instead.

GetIRawElementProviderSimpleFromTree was renamed to
GetIRawElementProviderSimpleFromId and a helper GetNodeFromTree method
was was added to to ax_tree_manager_base_unittest.cc, as that test
*does* look up nodes from a separate test tree (instead of using a tree
manager).

https://chromium-review.googlesource.com/c/chromium/src/+/5980301
will be re-purposed to delete GetNodeFromTree once this lands.

AX-Relnotes: n/a.

Change-Id: Ifde6cb9684cf6cb38de22b2fb4478e46a063c3d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5994673
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Jacques Newman <janewman@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1379427}