Add metrics for nav buttons through KeyButtonView

Doesn't cover home-long-press or overview, but those appear to be
covered by other events.

Test: runtest systemui
Merged-In: I590b52765aaef3c2bf573ce8ab980aa992b1d5b0
Change-Id: I630356245e223a3fb8d4d2fe11be5d498227c0e3
Fixes: 25931698