Use the current user rather than USER_SYSTEM as

default user for AccountManagerServiceShellCommand

This allows for testing of accountmanager with instant apps in secondary users.

bug: 138142372
Test:  cts-tradefed run cts  -m CtsAccountManagerTestCases -t android.accounts.cts.AccountManagerTest
$ adb shell am get-current-user
10
$ adb shell cmd account set-bind-instant-service-allowed true
$ adb shell cmd account get-bind-instant-service-allowed
true
$ adb shell cmd account get-bind-instant-service-allowed --user 0
false
$ adb shell cmd account get-bind-instant-service-allowed --user 10
true
$ adb shell cmd account set-bind-instant-service-allowed false
$ adb shell cmd account get-bind-instant-service-allowed --user 10
false

Change-Id: Ic4d4ce411f4471ea1b02e93f6a05857efc7287a8