Avoid integer underflow involving maxFreqCount.

If a device does not have any policy directories under
/sys/devices/system/cpu/cpufreq this would previously lead to the
cputimeinstate subsystem being initialized with an empty set of policy
frequencies. This would lead to integer underflows in various loops
that enumerate the frequencies when subtracting 1 from a maxFreqCount
variable calculated as 0, resulting in us spending a significant amount
of time in these loops, likely leading to an ANR in system_server
since at least the loop in clearUidTimes is executed while holding the
BatteryStatsImpl lock. Fix the problem by skipping the initialization
of cputimeinstate if there are no policy directories.

Bug: 142352330
Bug: 178231152
Change-Id: I2ec1e8de0fe2a40ed100c8f14e6ca3f6d6285b82