Migrate PrintActivity to enableOnBackInvokedCallback=true

Bug: 379263789
Bug: 376407910
Test: CtsPrintTestCases
Test: Manual, i.e. verified that PrintActivity has same back navigation behaviour with and without this CL applied. (tested within Files app after `m PrintSpooler; adevice update`)
Flag: EXEMPT bugfix
Change-Id: I53a34b448b520da7193eadadef09fc2cd99debbd