Change the default clock to Clock.systemUTC().

We were using SystemClock.currentNetworkTimeClock() for getting the
creation time of the stage data in LocaleManagerBackupHelper. This clock
cannot be modified by the user and hence the retention CTS tests were
failing. Because those tests manipulate device's system clock to
simulate passage of time.

Bug: 210593602
Test: atest FrameworksServicesTests:LocaleManagerBackupRestoreTest
Test: atest CtsBackupTestCases:AppLocalesBackupTest
Change-Id: Icc2e5d9f6035fa1e24382278f1ca45a6a6c4f32a