Add class to read per-thread CPU usage from proc filesystem

Reads all thread CPU usage for the current process. This traverses
"/proc/self/task/*/time_in_state" to gather the thread ID and CPU usage
of each child thread. Process name, thread name, and UID are also read from
"/proc" for interpretable data.

Bug: 111534779
Test: Unit test in CpuThreadProcReaderTest

Change-Id: I6c71dde1dfcc9bbb87d95baf0886f9da7a782299