Android uses android.os.SystemClock.uptimeMilles for event time and SystemClock.uptimeMilles uses SYSTEM_TIME_MONOTONIC.

So since TimeStamp posix impelemetation uses SYSTEM_TIME_MONOTONIC too, so we can use event time on FromSystemTime.

MozReview-Commit-ID: 5Qb5kmnHHCI

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extra : rebase_source : 26d1554fa60e3216edfd4b149380910fe2a9d845