Fix logspam from AppIdleHistory on first boot

Before /data/system/users/0/app_idle_stats.xml has been created, it is
expected that it doesn't exist.  Log a simple debug message instead of
an error message with stacktrace.

Bug: 290410617
Test: booted Cuttlefish and checked logcat
Change-Id: I19a72e3bd74038e7447a2d47529022a68a13b1e9