fix(dialog): default dialog actions alignment to end

[crisbeto]

Switches to defaulting the alignment for mat-dialog-actions to the end, rather than the start.

Fixes #13618.

[crisbeto]

Caretaker note: we should evaluate how many presubmit issues this causes before we decide whether to merge it in.

[mmalerba]

Closing since this fix is for the legacy mat-dialog which has been superseded by the MDC based one.

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