C-tor for SigninManagerImpl is private now and we can't extend it without patching. Since we don't use it, here we just add our own mock implementation of SigninManager interface. Chromium change: https://chromium.googlesource.com/chromium/src/+/1bdcfc90ebdb8085b0ce348db5c5461472e22835 [Signin][Android] Move operations out of SigninManager ctor This CL moves operations out of the SigninManager ctor to make the code safer, otherwise we could get back to the object with it only partially initialized. Bug: 1152333