This fixes a regression with an internal API from 27d1508 where
overridding the global bootstrap didn't impact parsing the default
bootstrap. So if no global bootstrap was available XdsNameResolver would
fail to start even though an override was in place in
SharedXdsClientPoolProvider. Instead of dealing with the override in
SharedXdsClientPoolProvider, do it in GrpcBootstrapperImpl so
XdsNameResolver is ignorant of the source of the default bootstrap.

We want all of this to go away in favor of XDS_CLIENT_SUPPLIER
injection, but there needs to be some overlap for migration.

cl/826085025

If the refcount goes negative, then the next getObject() will return
null. This was noticed during code inspection when investigating a
NullPointerException in b/454396128, although it is unclear if this is
actually happening.

…miss or stale cache entry (#12442)

b/348690462

Fixes #12168