Add GuardedBy annotations to policy engine

This will help find gaps in synchronization, but currently only fires
when code lines are changed. That means adding these annotations doesn't
necessarily surface all issues.

One can add javacflags: ["-Xep:GuardedBy:ERROR"] to enforce checks on
all lines.

Bug: 374511959
Test: m & Presubmit
Flag: EXEMPT only adding annotations
Change-Id: Ib7676dbdab7ff612610cc05580c656fba7484d3a