Calculate timestamp using local time zone in system update unit tests

Replace hardcoded timestamp for test dates with values calculated
from local time zone, because the SystemUpdatePolicy API assumes
dates and timestamps are in local time zone. Also tweak test dates
to avoid daylight saving time boundaries.

Bug: 74335646
Test: runtest frameworks-services -c com.android.server.devicepolicy.SystemUpdatePolicyTest

Change-Id: I532cd485a8ddd8b2a652fa90dee0deb932954662