Removes set/clearTestBrailleDisplayController APIs.

AccessibilityService callers of getBrailleDisplayController() can
instead use injection to replace the default BrailleDisplayController
with a fake for integration testing, rather than relying on a platform
API like this to set the replacement.

Bug: 326097646
Test: n/a
Change-Id: I0f9ff0182390debbc0952c8019a0cec7506a5044