Fix problems with new PowerManager.reboot() implementation.

ShutdownThread.reboot() does return so we need to block after calling it
to prevent PowerManager.reboot() from returning.

Since PowerManager.reboot() can now take significantly longer than before,
we now ignore ANRs during shutdown.

Change-Id: Ibceeb265ae382567215f6a399108d8be3a7bbc95
Signed-off-by: Mike Lockwood <lockwood@android.com>