Updating Java doc for Activity#onUserLeaveHint()

Currently, Activity#onUserLeaveHint() may not have clear enough
information descripting when it would be called. Updating
document to also check Intent#FLAG_ACTIVITY_NO_USER_ACTION
for reference.

Bug: 121282783
Test: m -j PRODUCT-sdk_x86-sdk
Change-Id: I3894efbe286575140afbbba2ed059fd0f129ef3b