Use correct call to get Android user ID

Rather than trying to store the active user ID, use
multiuser_get_user_id to pull the actual ID for the caller. This will
now behave correctly when called by background processes.

Test: CtsKeystoreTestCases, from both owner and guest users
Bug: 113280004
Change-Id: I73fd2ef2ac4b1ceb7ec3792761e827d801d0d905
diff --git a/keystore/Android.bp b/keystore/Android.bp
index 9bf6cf3..ed60596 100644
--- a/keystore/Android.bp
+++ b/keystore/Android.bp
@@ -50,6 +50,7 @@
         "libbase",
         "libbinder",
         "libcrypto",
+        "libcutils",
         "libhardware",
         "libhidlbase",
         "libhidltransport",