[pm] set firstInstallTime for system apps on createNewUser

(As a follow up on ag/25951290)

This field used to be 0 (effectively the beginning of unix time) so it's
not very useful. Set it to the actual timestamp when the system app is
marked as installed on the new user.

BUG: 322246299
Test: presubmit

Change-Id: I15a638c83032a086db5d413ada6289692d941490