init: Add diagnostics for snapuserd hangs

This patch attempts to diagnose snapuserd hangs by performing reads
immediately after entering second-stage init. This is done by spawning
two threads: one to perform the reads, and another to wait for the read
thread to finish. If any aspect of the read fails, or the read thread
does not complete in 10 seconds, then a list of snapuserd's open file
descriptors are logged.

Bug: 207298357
Test: apply working OTA, check logcat for success
      apply broken OTA, check logcat for fd map
Change-Id: I549e07b7d576fcdaca9b2d6ff33e0924c3812c07