Skip dexbootjar check using AllowMissingDependencies flag

Soong checks that every module part of a bootclass_fragment provides a
dex boot jar file, even if the module is not a dependency of vendor.img

Using AllowMissingDepdencies() to skip this check allows vendors to be
more aggressive in removing projects from their source tree

Test: In build/soong, run go test ./...
Test: m nothing
Bug: 196306898
Bug: 192616764
Change-Id: I78b062afdc19a6a3251aa8552230f3fcf334b6fb