Move EntropyMixer I/O from main thread to I/O thread

This moves EntropyMixer I/O from the main thread to the I/O thread to
avoid blocking the main thread. The only time EntropyMixer performs
I/O on the main thread is during initialization of EntropyMixer which
occurs only during startup of SystemServer.

Test: manual: locally modified writeEntropy to log thread name and
      observed that writeEntropy is invoked only on the I/O thread
      ("android.io") except when invoked by EntropyMixer constructor.
Test: atest com.android.server.EntropyMixerTest
Bug: 70471586

Change-Id: Ic9f07f1c65583bb57d3095f9f8e7dde4ea6ae07d