Allow DiskRead ThreadPolicy in LetterboxConfigurationPersister

The LetterboxConfigurationDeviceConfig needs to access the
file with the current reachability position once when the
device boots. Because DisplayThread uses allowIo=false
accessing a file triggers a DiskReadViolation.
Here we use StrictMode to allow the current thread to read
the AtomicFile once in the current thread restoring the
original ThreadPolicy after that.

Fix: 286620565
Test: Run `atest WmTests:LetterboxConfigurationDeviceConfigTests`

Change-Id: Ia358de8c817eea987547c82df0ea8612591c0600