[Task] Vary live region politeness based on `priority`

[smockle]

From w3c/aria#2577 (review):

[The polyfill] always uses a polite live region (rather than varying based on priority).

The polyfill should conform to this, from w3c/aria#2577’s CORE-AAM spec draft:

Let mapped_priority be Atk.Live.ATK_LIVE_ASSERTIVE if priority is "high", otherwise Atk.Live.ATK_LIVE_POLITE.
[…]
Let mapped_priority be ATSPI_LIVE_ASSERTIVE if priority is "high", otherwise ATSPI_LIVE_POLITE.