Add temporary workaround for adding threads to RpcServer

We are unable to add new systemapi NDK symbols at API level 36.1 so
AIBinder_setMinRpcThreads is unavailable. This sets max threads for the
com.android.isolated_storage_service.IIcingSearchEngine binder when
creating a new vsock server in libbinder_rpc_unstable.

Flag: EXEMPT Bug fix
Test: atest MicrodroidTestApp
Bug: 430314963
Change-Id: I1089db93f8f4129da63eb0a8300bd459b6fa497f