fix crash on invalid base64 key

An invalid adb public key can cause a system server crash.

  AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: android.fg
  AndroidRuntime: java.lang.IllegalArgumentException: bad base-64
  AndroidRuntime:        at android.util.Base64.decode(Base64.java:161)
  AndroidRuntime:        at android.util.Base64.decode(Base64.java:136)
  AndroidRuntime:        at com.android.server.usb.UsbDebuggingManager.getFingerprints(UsbDebuggingManager.java:235)
  AndroidRuntime:        at com.android.server.usb.UsbDebuggingManager.access$500(UsbDebuggingManager.java:49)
  AndroidRuntime:        at com.android.server.usb.UsbDebuggingManager$UsbDebuggingHandler.handleMessage(UsbDebuggingManager.java:210)
  AndroidRuntime:        at android.os.Handler.dispatchMessage(Handler.java:102)
  AndroidRuntime:        at android.os.Looper.loop(Looper.java:135)
  AndroidRuntime:        at android.os.HandlerThread.run(HandlerThread.java:61)
  AndroidRuntime:        at com.android.server.ServiceThread.run(ServiceThread.java:46)
  Process : Sending signal. PID: 824 SIG: 9

Bug: 17389885
Change-Id: Id20d7c0ee35229b14c800fd9e07716239179989c