contexts: fix signature of overridden function

The new version of Clang emits errors on this code, since the function
being overridden returns a `const keymaster_algorithm_t*`, but the
overriding function specifies `keymaster_algorithm_t*`.

BUG=b:407993702
TEST=FEATURES=test emerge-brya arc-keymint

Change-Id: Ic7d29a77d6e27018240ed6c9df1c31a01301804c
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/keymaster/+/6427205
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Vaibhav Raheja <vraheja@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>