Fix SQLitePersistentCookieStore's getDbThread() to be threadsafe

This is an updated version of https://android-git.corp.google.com/g/#/c/141899,
which was rolled out, with the lock now static and moved to file scope to match
Chromium style.

Bug: 5244039
Change-Id: Ic6a6e949b2adc8cf433e6468c1f84ed449af09bc