Simplify Context Hub Service transaction locking

This CL simplifies the locking scheme used in the ContextHubTransactionManager to avoid granular per-transaction locks to avoid potential race conditions (for example a response and a timeout occurring at the same time, triggering duplicate calls to transaction.setResponse).

Bug: 433459445
Flag: android.chre.flags.simplify_service_transaction_lock
Test: CHQTS pass

Change-Id: I8232b1ccb153df5639186d12b3c8ff77a00ffc6b