Distinguish unset system properties from errors reading a property.

Also expose error enum rather than wrapping it in anyhow, so users of
the API can match on the error variants and handle them as they wish.

Bug: 217728265
Test: adb shell /apex/com.android.virt/bin/vm info
Change-Id: Icff27ab12b52e7eff74b522f3841f50f86e982fc
1 file changed