title: Android device vendor system signature key uniqueness
body: <p>I'm sure this is a dumb question but Googling got me nowhere.</p>
  
  <p>Does each device vendor do its own Android build? More to the point, does each device vendor sign its build with its (the vendor's) own system signature key, such that we should not expect any two Android devices (at least, devices from different vendors) to utilize the same system signature key?</p>
  
  <p>Example: I know I'm not the first person to want to run an app with sharedUserId="android.uid.phone" and process="com.android.phone". But this requires my app to have been signed with the system signature key. If, by some miracle, I were able to obtain the system singnature key for my Acer device and sign my app with it, would this restrict my app to run on Acer devices (or perhaps just my particular model), since other vendors' devices would have their own Android builds and their own system signature keys?</p>