Wire the new FooterActions implementation behind a flag

This CL wires the new footer actions implementation from ag/19678215
behind a new SysUI flag. This flag will be enabled by default in a
follow-up CL.

Bug: 242040009
Test: atest FooterActionsViewModelTest
Test: atest FooterActionsScreenshotTest
Change-Id: If23711e400a86ad247666a1532cb17616c01ed85