Invalidate the CSS token after default duration (10 minutes) to improve security.

See go/l2a-css. This change also adds an adb command to set the token
validity duration for testing purposes. The duration cannot exceed the
default token duration.
`adb shell cmd contextual_search set token-duration 200000`

Bug: 338598282
Test: CTS and manual tests
Change-Id: I0f996f5f6f3eaaf12900dbebd44d9c8d04308f15
(cherry picked from commit 7de3d5d3cdc7c516edcf0062c9a3ca4acee9bce1)

Modify ContextualSearchService to refresh token in the callback.

Checks the enableTokenRefresh aconfig flag to determine if CSS should
call the callback with a new token each time getContextualSearchState
is called. If true, the callback will be called twice:
- Once with a fresh token for the next getContextualSearchState call.
- A second time with the assist state.

FLAG:android.app.contextualsearch.flags.enable_token_refreshBug: 338598282
Test: CTS and tested on device
Change-Id: I73c2db2758b396482d1f4e78d50febf0606eb8ad