bootstat: reduce overall boot time

Run bootstat as a oneshot service rather than as a series of inline
exec commands.  exec commands impact boot time.

Test: cts-tradefed run cts-dev --module CtsBootStatsTestCases
      system/core/bootstat/boot_reason_test.sh all
         (make sure it filters out new init reports)
Bug: 65736247
Change-Id: Ic9d509a8cbee4bc1e278081de1001e25ae0915fd