FTL: Add wrapper for shared_mutex

std::shared_mutex on Android is missing capabilities necessary for
threading annotations. b/135688034 tracks adding them, but is blocked on
b/175635923. In the meantime, add a simple wrapper so that we can start
using annotations.

Only add the methods we currently need. Other methods can be added as
needed.

Bug: 185536303
Test: ftl_test
Change-Id: Ic7c2152bc7e46b31eecdba42fac1126b26aafd60