Adds TEST_MAPPING for Settings > Accessibility unit tests.

This initial TEST_MAPPING file is postsubmit only until we validate
which tests are stable enough to be moved to presubmit and which need
fixes.

Bug: 267225211
Test: atest --test-mapping packages/apps/Settings/src/com/android/settings/accessibility
Change-Id: I5e5f337d4ea7618ab92d3daa5171fd8ee1508c59