Convert BuildIgnoreApexContritbutions variable to a boolean

The ignore list is burdensome to maintain once we start adding the
module sdk contents to apex_contributions. Convert the variable to a
boolean. When set to true, all contents in `apex_contributions` will be
ignored

Bug: 308187268
Test: m nothing on aosp,google and google_fullmte devices
Ignore-AOSP-first: CL topic does a cleanup of an internal only denylist

Change-Id: If899f6eaf5449c2aa789d0bd5b791a3db715c676