Fully deprecate Process.setCanSelfBackground

Make this method a no-op, preserved only to avoid app compat breakage.
There's no evidence this has been used in recent history in practice,
and by default its behavior is behind a disabled-by-default #define.
All it does is add slight JNI overhead for certain common code paths.

Bug: 441066069
Flag: EXEMPT REFACTOR
Test: presubmit
Change-Id: I37d09edd33e16d5443bc35373a05b17908a62fdc