liblog: Always report as debuggable when building userdebug/eng

This doesn't affect the normal behavior of our builds, because
ro.debuggable is set to 1 on userdebug/eng anyway.

However, it's helpful because making __android_log_is_debuggable
return a compile time value rather than the value of runtime prop
lets us pass checks that'd otherwise prevent us from using adb root
when Magisk is installed.

Change-Id: I36f53976162e652a38008ced459ca02fd6c0af51