Allow uid to be passed for more operations

This expands get, getmtime, exportKey, getKeyCharacteristcs and begin to
accept a uid to run as. This is only for system to use keys owned by
Wifi and VPN, and not something that can be used to do operations as
another arbitrary application.

Bug: 23978113
Change-Id: If076d61b0cc9d55e96272e49a58938c3961e2dda
diff --git a/keystore/keystore_get.cpp b/keystore/keystore_get.cpp
index 45ad415..2783816 100644
--- a/keystore/keystore_get.cpp
+++ b/keystore/keystore_get.cpp
@@ -31,7 +31,7 @@
     }
 
     size_t valueLength;
-    int32_t ret = service->get(String16(key, keyLength), value, &valueLength);
+    int32_t ret = service->get(String16(key, keyLength), -1, value, &valueLength);
     if (ret < 0) {
         return -1;
     } else if (ret != ::NO_ERROR) {