Resolve STATSD and batterystats race condition

Both STATSD and batterystats need to read uid cpu info. However, uid cpu
stats needs to be cleared from time to time to conserve memory. To
resolve this race condition, only batterystats will remove uid stats,
both from readers and from the kernel, also with a delay, so that STATSD
can access such info before it is removed.

Refactored readers to reuse some common code. Also removed string reader
from KernelUidCpuFreqTimeReader completely since binary reader has been
working fine for a while.

Change-Id: I209bdcec642e1a29a44b566ce98ebbfaaacb4e6a
Fixes: 72172569
Test: BatteryStatsCpuTimesTest
Test: KernelUidCpuActiveTimeReaderTest
Test: KernelUidCpuClusterTimeReaderTest
Test: KernelUidCpuFreqTimeReaderTest