refactor: change isDateInstance return type to is D

[DevTrong]

Change return type of DateAdapter.isDateInstance from boolean to obj is D

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[DevTrong]

@googlebot I fixed it.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[mmalerba]

@DevTrong can you please run this command to update the API golden file:

yarn bazel run //tools/public_api_guard:material/core.d.ts_api.accept

[mmalerba]

LGTM

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