Indent the word 'Proof' without using xpatch?

I can’t figure out how to work around this problem. I need to indent the word ‘Proof’ in the proof environment of amsthm without using the xpatch (xpatch is described in here Indent the word “Proof” in environment proof of amsthm).

I want to avoid using any patches because they might stop working in the future. I want to do this using amsthm package only.

This is the minimum working code:

documentclass[12pt,a4paper]{book}
usepackage{amsthm}
usepackage{indentfirst}
usepackage{lipsum} 

newtheoremstyle{sltheorem}
{2pt}                % Space above
{-2pt}                % Space below
{it}        % Theorem body font % (default is "upshape")
{17pt}                % Indent amount
{scshape}       % Theorem head font % (default is mdseries)
{.}               % Punctuation after theorem head % default: no punctuation
{ }               % Space after theorem head
{Theorem thetheorem}                % Theorem head spec
theoremstyle{sltheorem}
newtheorem{theorem}{Theorem}[chapter]

begin{document}

chapter{one}
lipsum[2]

begin{theorem}
lipsum[2]
end{theorem}

begin{proof}
lipsum[2]
end{proof}

end{document}