Only open acpu_stats file once.

This means we can just seek & read in the main loop. This speeds up
feature collection 2x.

Bug: 207662659
Test: atest libadaptivecpu_tests:KernelCpuFeatureReaderTest
Change-Id: Ie9259ffea19bb9d10196b9ced5543581e1f80734
7 files changed