Add dropdown caret to time owner dropdown on Date and Time Settings

- Use background-image strategy to add caret to dropdowns
- Standardize implementation by including mixin to dropdown__button
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Add dropdown caret to time owner dropdown on Date and Time Settings

- Use background-image strategy to add caret to dropdowns
- Standardize implementation by including mixin to dropdown__button
class and removing styled pseudo element

Resolves ibm-openbmc/dev#478

Tested: Running locally on Chrome only. Ran into network request
errors for newly added svg assets when mounting overlay

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: I714c2515282676f368eafb2560177d2935680e23

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