fix(calendar, datepicker): fix issues with GMT+X timezones

[Splaktar]

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

When local timezone is GMT+X, (i.e. +5:00) using ng-model-options to specify the timezone causes the previous day to be selected.

Issue Number:
Fixes #12000

What is the new behavior?

• fix issues with GMT+X timezones by avoiding timezone conversions when local timezone is GMT+x. We let the existing utilities for converting dates to midnight handle things since the date doesn't need to change in those cases, just the hours.
• simplify $$mdDateUtil.removeLocalTzAndReparseDate()

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

We don't have the capability to change the device's timezone between tests.

[wagnermaciel]

lgtm