Fixing empty PendingAlarmInfo after alarm store changes

The alarm store implementation might change, leaving the pointer passed
to MetricsHelper with an empty instance.

Also reordered some code to reduce unnecessary locking.

Test: Manually, run:
adb shell device_config put alarm_manager lazy_batching false
Then:
./out/host/linux-x86/bin/statsd_testdrive 10106
should show non-zero values.

Bug: 177556103
Change-Id: I55b4318d14632f227adfd79f7a08bb50af98f03a