feat(cdk/a11y): add TreeKeyManager to public a11y API

Author: BobobUnicorn

src/cdk/a11y/public-api.ts

@@ -10,6 +10,7 @@ export * from './aria-describer/aria-reference';
 export * from './key-manager/activedescendant-key-manager';
 export * from './key-manager/focus-key-manager';
 export * from './key-manager/list-key-manager';
+export * from './key-manager/tree-key-manager';
 export * from './focus-trap/configurable-focus-trap';
 export * from './focus-trap/configurable-focus-trap-config';
 export * from './focus-trap/configurable-focus-trap-factory';