Fix cumulative execution calculation.

The base timestamp wasn't being reset when the job started, so the
duration was always being perceived as starting from boot time. Setting
the base timestamp as the job start time fixes the cumulative execution
tracking.

Bug: 280653695
Test: N/A
Change-Id: I8fdcde54c21bc53cfdb41c78ba4f19d7b45cc50b

apex/jobscheduler/service/java/com/android/server/job/JobServiceContext.java

@@ -350,6 +350,7 @@ public final class JobServiceContext implements ServiceConnection {
                     job.shouldTreatAsUserInitiatedJob(), triggeredUris, triggeredAuthorities,
                     passedNetwork);
             mExecutionStartTimeElapsed = sElapsedRealtimeClock.millis();
+            mLastExecutionDurationStampTimeElapsed = mExecutionStartTimeElapsed;
             mMinExecutionGuaranteeMillis = mService.getMinJobExecutionGuaranteeMs(job);
             mMaxExecutionTimeMillis =
                     Math.max(mService.getMaxJobExecutionTimeMs(job), mMinExecutionGuaranteeMillis);