ImageDecoder: Add getters. Rename setAsAlphaMask

Bug: 73788928
Test: I1606cbb4e71579160ffaef12c1ed738fad882cd1

This will allow Kotlin developers to treat the setters as properties.

Part of the motivation for the rename is that "getAsAlphaMask" sounds
like it returns an alpha mask itself. Rename both to
_etDecodeAsAlphaMask.

Change-Id: I8f9b04f8381840490b662c3bcd37a95442af8110