Suppress ThreadNetwork HAL debug log on user build

Due to security concerns, ThreadNetwork HAL service's debug log should
not be visible on user builds.

The "debuggable" field defines the flags that will be applied to
userdebug/eng variant builds.

Bug: 359338609
Test: No debug log from android.hardware.threadnetwork-service in logcat
Change-Id: I13993191efd3841fc8db1888be7c59c86f6608d3