Add PendingRequestPool to handle pending requests.

PendingRequestPool would store all pending requests that we have
not yet got responses from hardware. If a request has been pending
for too long, the timout callback would be called and the request
would be removed.

Test: atest DefaultVehicleHalTest
Bug: 203713317
Change-Id: I4d7ae2c72b960347be70ac4cc8ce3d66eb8128f9
diff --git a/automotive/vehicle/aidl/impl/vhal/Android.bp b/automotive/vehicle/aidl/impl/vhal/Android.bp
index 454fea5..a54ab4b 100644
--- a/automotive/vehicle/aidl/impl/vhal/Android.bp
+++ b/automotive/vehicle/aidl/impl/vhal/Android.bp
@@ -56,6 +56,7 @@
     srcs: [
         "src/ConnectedClient.cpp",
         "src/DefaultVehicleHal.cpp",
+        "src/PendingRequestPool.cpp",
     ],
     static_libs: [
         "VehicleHalUtils",