Update duration calculation in AppProfiler

This change replaces currentThreadTimeMillis() calls in AppProfiler's
collectPssInBackground/collectRssInBackground with uptimeMillis(). This
should be less expensive, while still calculating the same duration.

Test: build, flash, grab bug report and verify that dumpsys procstats
      still contains PSS/RSS duration info
Bug: 296454553
Change-Id: I0c5f59c99e1e0bc71a42c0ea887b1a0a32c5dbd6