Reduce to hold mUsersLock for the system user

If the caller is the system, the exists method avoid calling to
LocalService.exists, which can reduce the holding of mUsersLock

This is just an optimization of using this method for PMS.

Bug: 410481999
Flag: EXEMPT bug fix
Test: atest CtsPackageInstallTestCases
Test: atest CtsPackageManagerTestCases
Change-Id: Ifc7362aa6ca04ae0b25ccbae90d8ab9486737db5