Use METHOD value instead of SIGN_UP_METHOD

[ksheremet]

SIGN_UP_METHOD was deprecated. Use METHOD in logSignUp
https://firebase.google.com/docs/reference/android/com/google/firebase/analytics/FirebaseAnalytics.Param#SIGN_UP_METHOD

[googlebot]

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[ksheremet]

I signed it!

[googlebot]

CLAs look good, thanks!

[ksheremet]

Looks like analyzer failed for an unrelated reason (dart-lang/linter#834).

[ksheremet]

Rebased to latest, all tests pass now.

[ksheremet]

A friendly ping! +cc @kroikie

[ksheremet]

Another friendly ping!