Mark some functions in InputReader code as const

This helps us reason about their side-effects.

Bug: 378308551
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST --gtest_filter="*ChangeAssociatedDisplayIdWhenTouchIsActive*"
Flag: EXEMPT refactor
Change-Id: I0a2e34b066254833c1fdc03c8dd3eee0962f979f
14 files changed