Group child sessions together with parent in output of pm get-stagedsessions

This makes relation between parent and child sessions easier to spot.

Also it unveiled that surprisingly some of sessions have child sessions
unknown to PackageManager. Will debug this.

Example output:
sessionId = 945170441; appPackageName = null; isStaged = true; isReady = false; isApplied = false; isFailed = true;
  sessionId = 381169012; not found
  sessionId = 1180853507; not found
sessionId = 1109749804; appPackageName = null; isStaged = true; isReady = false; isApplied = true; isFailed = false;
sessionId = 1870384217; appPackageName = null; isStaged = true; isReady = false; isApplied = true; isFailed = false;
  sessionId = 433320004; appPackageName = null; isStaged = true; isReady = false; isApplied = false; isFailed = false;
  sessionId = 865092704; appPackageName = null; isStaged = true; isReady = false; isApplied = false; isFailed = false;
sessionId = 2030351614; appPackageName = null; isStaged = true; isReady = false; isApplied = false; isFailed = true;
  sessionId = 497536355; not found
  sessionId = 1079410796; not found

Test: installed staged session & adb shell pm get-stagedsessions
Change-Id: I3c8d9590814a4246d85d52c189ad5abd5f72c9cf