Update shell command line to take optional user id.

Previously a user id is required when `adb shell cmd game mode` is
called. This patch modifies it to be optional and use the current active
user id as the default user id when not specified.

Minor: Fix multi-user issue in GameManagerService when querying package
uid.

Bug: b/190561852
Test: atest GameManagerService
Change-Id: I1e21ee8e08930fc5ac5d12a13b2aeab09335a5b6