Translate: API for content padding.

This allows controlling the content padding behavior needed with some
apps for compatibility reasons.
(see I9d7db5d2bc200e4042baf3267796f00cfc298f19)

The flag is currently the only property in the new UiTranslationSpec
value type, but more properties can be added in the future (such as
animation specifications).

Bug: 179693024
Test: manual
CTS-Coverage-Bug: 182990474
Change-Id: I11052cddb59c928b22ad9ad16e3c9b092da4b907