Make UntrustedSpy_AbortsDispatcher threadsafe

A similar fix worked for the other dispatcher death test. The test
UntrustedSpy_AbortsDispatcher is a lot less flakier, with a fail rate of
about 1 in 1502.

Still, we want to achieve a near-zero flake rate, so apply the same fix
here.

Bug: 291797922
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: Ic4a759862f110a24f7b239b9e20fa0222fd6668d