Be more explicit about users

Be more explicit about users executing processes in the time
zone updates code.The code was already running everything as the
system user but now that's more explicit / by design.

To service unit tests:
make -j30 FrameworksServicesTests
adb install -r -g "${ANDROID_PRODUCT_OUT}/data/app/FrameworksServicesTests/FrameworksServicesTests.apk"
adb shell am instrument -e package com.android.server.timezone -w com.android.frameworks.servicestests \
  "com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner"

Bug: 64111659
Test: Unit tests: See above
Test: Manual testing installing updates as secondary device user
Test: PTS: run pts -m PtsTimeZoneTestCases
Change-Id: Idb754f3e1aa3830ba1ada8ef5740f9f7340f03d5