Fix rotation bug in Bluetooth pairing PIN dialog

BluetoothPairingDialogFragment has code that makes the OK button on the
dialog disabled until the user has entered at least one character into
the PIN field. However it didn't properly handle the case where the user
had entered some text and then rotated the screen - because it always
marked the OK button as disabled during onShow even if it already had
some content. This CL fixes that by looking at the text content and only
disabling the OK button if the content is empty.

Bug: 36514895
Test: make RunSettingsRoboTests
Change-Id: I4e8e70089a862e67b20ff614bbaa64fc2b641fd4