Merge "Document that this ResolverActivity isn't directly used." into udc-dev am: 45246ffd7d

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/IntentResolver/+/23209030

Change-Id: Id3ab4bcd0bb450adbbfa74741f7bb40fda9e204f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>