Use the camera override only if it resolves

If it's nonexistent (bad configuration), uninstalled, hidden, or
disabled, we shouldn't honor the override.

Test: atest PlatformScenarioTests:android.platform.test.scenario.sysui.lockscreen.LaunchCameraFromLockScreen
Test: watch --interval=0.1 'adb shell dumpsys activity service SystemUIService  | egrep -A4 Camera.*intent'
Bug: 315698413
Bug: 235154275
Change-Id: I97df575b09342fc6a3da0c1fc9c47e785a8f75f6