Configurable sampling rate for hidden API access log events.

To reduce log spam, we do not log all hidden API accesses. This CL allows
configuration of the sampling rate of the log events so it can be tweaked
later if necessary.

Test: m
Test: $ adb shell settings put global hidden_api_access_log_sampling_rate 65536
Bug: 64382372
Bug: 77517571
Change-Id: I659c22bd504564da58d972f94b774a9af4039386