Camera2OneCameraManagerImpl: fix NPE when only one camera is available

* AOSP 10 introduced a crash on devices with only one camera that was
  not present on AOSP 9. This was introduced in commit 8b07da, which
  added cache to the result of `findCameraId()`, but did not check if
  the value is null before adding to cache. Code elsewhere checked the
  result of `findCameraId()` to avoid NPEs, but the function itself
  would crash if it were ever to return null due to this regression.

* Fixed by simply wrapping the addition to cache in a null check.

Change-Id: I01730cf6d2ffb7a82cc243de7313f1a955afd6bf
1 file changed