Add getFdStateDebug to access Looper's callbacks

Added getFdStateDebug to access Looper's internal state.

Flag: EXEMPT testing
Test: TEST=libutils_test; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST --gtest_filter="LooperTest.getFdStateDebug*"
Change-Id: I253ed4a6fa1040053117dcea3be80e55eef9a9b0