Instantiate PasswordStoreEmptyBackend when internal backend missing

Before this CL:
1) Internal backend missing -> Null password stores
2) Internal backend present but GmsCore outdated -> Non-null password
   stores with PasswordStoreEmptyBackend
3) Otherwise -> Non-null password stores with
   PasswordStoreAndroid{Account,Local}Backend

After this CL:
1) is changed to do the same as 2). That's one less product state.

Bug: None
Change-Id: I95eba78d985ef371bb515f11e40e2e4ba7bdc145
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6909110
Auto-Submit: Victor Vianna <victorvianna@google.com>
Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1510853}