Align boot jar exclusion with apex_contributions

During trunk stable development, it is possible that a new boot jar is
added to the source apex, but not available in prebuilt apex yet.
Thefore we need to conditionally check the `contents` of the respective
bootclasspath_fragment modules. https://r.android.com/2822393 did this
using isActiveModule. This function relies on soong config namespaces to
"hide" the source bootclasspath_fragment module when
<module>.source_build is false.

Soong config namespaces for mainline source vs prebuilt selection is no
longer in use, so this CL replaces `isActiveModule`. The CL cleaves the
implementation
1. For source builds, the check will run in the context of the
   bootclasspath_fragment module. `disableSourceApexVariant` will be
used to skip the check if prebuilts are selected.
2. For prebuilt builds, the prebuilt bootclasspath_fragment module sets
   a provider, and the top-level apex will do the check.

(2) is necessary to handle the edge case of multiple versioned mainline
prebuilts and possible skew in apex boot jars across them. In case of
prebuilt builds, the unselected mainline prebuilts will have
HideFromMake called on them, so the check will only run on the apex that
is actually used during the build.

Bug: 328578801
Test: go test ./apex

Change-Id: I6eec603397eea926f3a481b79ca938245064d809
Merged-In: I6eec603397eea926f3a481b79ca938245064d809