base: work around thread safety analysis bug.

Clang's assert_capability and assert_shared_capability annotations are
nonfunctional until https://reviews.llvm.org/rL309725 is relanded and
makes its way into our toolchain. Work around this by using the
equivalent assert_lock and assert_shared_lock.

Bug: http://b/64226736
Test: manual
Change-Id: I40711f162ea1d492f1e0b3eff88bf6ae6d995e2f