Make assertPosition a method of FakePointerController

This way it can be used in tests that aren't based on InputMapperTest.

Bug: 251196347
Test: m inputflinger_tests && \
    $ANDROID_HOST_OUT/nativetest64/inputflinger_tests/inputflinger_tests \
    --gtest_filter='*HardwareStateConverterTest*'
Test: atest inputflinger_tests
Change-Id: I39a7f08ac9b705d12e45acc3e3773b0046e80e5c