Fix two android metrics reporting in update engine

1. The AttemptCurrentBytesDownloadedMiB was reported in bytes.
2. We were using the same start timestamp (in monotonic_time) for both
   boot_time and monotonic_time. This leads to an inaccurate update
   duration reporting. This cl saves an extra perf for the boot time and
   calculates the duration accordingly.

Also report these information with a different metrics name.

Bug: 30989466
Test: Run an update and check the report
Change-Id: I69e20648501a736a59ff60ca9202ce7bdd9b6e34