Clear temporary directory on start.

This is necessary in case virtualizationservice crashes and restarts for
some reason, and is also simpler for permissions than clearing it on
boot.

Bug: 192917795
Bug: 193365943
Test: created files, restarted virtualizationservice
Change-Id: I6a30517a76830c2440df268b45631bf9bffc9474
diff --git a/virtualizationservice/src/aidl.rs b/virtualizationservice/src/aidl.rs
index 661abdc..d99d63a 100644
--- a/virtualizationservice/src/aidl.rs
+++ b/virtualizationservice/src/aidl.rs
@@ -52,7 +52,7 @@
 pub const BINDER_SERVICE_IDENTIFIER: &str = "android.system.virtualizationservice";
 
 /// Directory in which to write disk image files used while running VMs.
-const TEMPORARY_DIRECTORY: &str = "/data/misc/virtualizationservice";
+pub const TEMPORARY_DIRECTORY: &str = "/data/misc/virtualizationservice";
 
 // TODO(qwandor): Use PermissionController once it is available to Rust.
 /// Only processes running with one of these UIDs are allowed to call debug methods.