Set accessibility action label for smart replies.

Currently the TalkBack reads something like "I am here, button,
double-tap to activate". Set an accessiblity delegate that changes
the click action label to "send", so the TalkBack message becomes
"I am here, button, double-tap to send".

Bug: 72216277
Test: In Notify post a notification with choices. Read it via TalkBack.
Change-Id: Ib50f6ff6f698196d905aea2e303b6bd2e5531587