Lock accesses to mClazz in AIBinder with a mutex

The associateClass method mutates the mClazz member of AIBinder in a
non-thread-safe way. This patch fixes this problem by adding a
std::mutex locking that variable, fixing test flakiness in the Rust
tests for the AIDL compiler

Test: atest libbinder_ndk_unit_test binderVendorDoubleLoadTest CtsNdkBinderTestCases
Test: atest aidl_integration_test for AIDL Rust tests
Change-Id: I4b0ec21755a9995c1f591ab937e9a3280992be80