Fix potential crash when per-procstate cpu times tracking is turned on.

When per-procstate cpu times tracking is turned on,
BatteryStatsImpl tries to access mKernelSingleUidTimeReader but
it's possible that mKernelSingleUidTimeReader hasn't been initialized
yet after a reboot and this could lead to a system_server crash.

Bug: 111523951
Test: manual
Change-Id: Id014f23fbe31fed64fba769f14ba4396a003092e
Merged-In: Id014f23fbe31fed64fba769f14ba4396a003092e
(cherry picked from commit 020239df)