Fix soundness issue in keystore2.

Sync was incorrectly implemented for AuthRequest, allowing simultaneous
access to a Receiver from multiple threads despite it not being
threadsafe. Use a Mutex instead to do this safely.

Bug: 290018030
Test: m rust
Change-Id: I6f43f13d5f36bdbafc9bd910a1ebadbb1366009d
1 file changed