[Bugfix] Keep initially set values of beginDate and endDate for datepicker

[AVykhrystyuk]

Fix cases when datepickerInput._valueChange event happens earlier than @input() being set for rangeMode

Steps to reproduce the issue:

• add the following markup

<input matInput [matRangeDatepicker]="rangePicker" formControlName="propertyWithDateRange">
<mat-range-datepicker #rangePicker [rangeMode]="true"></mat-range-datepicker>
<mat-range-datepicker-toggle matSuffix [for]="rangePicker"></mat-range-datepicker-toggle>

• initialize propertyWithDateRange in the component constructor or use a property-initializer:

public propertyWithDateRange = { begin: new Date(), end: new Date() }

• open date picker and click Cancel
  Actual result: Date field is empty
  Expected result: selected date is not changed when a user clicks on Cancel in date picker