[DLC vNext] Guard usage of SystemProperty with Build.isDebuggable

I found only 2 places where reading a SystemProperty is not guarded by
Android build type check. There is another class
com.android.devicelockcontroller.debug.DeviceCheckInClientDebug, but it
is NOT used anywhere and it's clearly for debug purposes only. So I did
not change it.

Bug: b/290932848

Test: m RunDeviceLockControllerRoboTests
Change-Id: Ib2ff7ece0cf8d33b57bba6e7c0648b8a1d26b170
2 files changed