[Contextual Edu] Check if target device is connected before incrementing signal count

Only increment signal count when the target device is connected.
e.g. As we will provide touchpad education for BACK gesture, only increment signal count when touchpad is connected.

Bug: 362495235
Test: KeyboardTouchpadStatsInteractorTest
Flag: com.android.systemui.keyboard_touchpad_contextual_education

Change-Id: Ibe57a2497714c8d880a7df581d5802ea011842b5