Use bpf data when available for single-UID cpu stats

Update KernelSingleUidTimeReader to read BPF data rather than per-UID
proc files when the BPF data is available. This is implemented by
calling libtimeinstate functions via JNI.
Extend KernelSingleUidTimeReaderTest to exercise both the BPF and proc
file code paths.

Bug: 138317993
Test: KernelSingleUidTimerTest passes
Test: no regression in BatteryStatsTests
Change-Id: Ie962ddd9e30d96aa0fab6104a4164af9ad02f55e
Signed-off-by: Connor O'Brien <connoro@google.com>