Workaround to make AlwaysUsePrebuiltSdks() work with platform_bootclasspath

The AlwaysUsePrebuiltSdks() causes all java_sdk_library_import modules
to be preferred over the source, i.e. as if they had prefer: true set.
That interacts badly with the work that is being done to integrate the
bootclasspath_fragment/platform_bootclasspath modules into the build.

It would work fine once that integration has been completed but in the
interim it causes problems. e.g. it does not cause a problem in AOSP
because those java_sdk_library_import modules that are affected have
already been integrated into the build properly.

Unfortunately, internally that is not the case because there are
java_sdk_library/java_sdk_library_import modules that still need to
be updated.

Before the java_sdk_library_import can be safely preferred each
java_sdk_library/java_sdk_library_import module that contributes to the
bootclasspath must:
* Be in the contents of matching bootclasspath_fragment and
  prebuilt_bootclasspath_fragment modules.
* Have an apex and one of a prebuilt_apex/apex_set that contains the
  dex implementation jar and lists the prebuilt_bootclasspath_fragment
  name in its exported_bootclasspath_fragments property.

Safely preferred in this context means that the whole build will
continue to work rather than the current situation which is that only
some of the build will work and some will fail if an attempt is
actually made to build it.

Unfortunately, many java_sdk_library_import modules are missing:
* The prebuilt_bootclasspath_fragment.
* The exported_bootclasspath_fragments property on the
  prebuilt_apex/apex_set that contains them.

Together these cause the following symptoms:
1. The java_sdk_library_import does not have a dex implementation jar.
2. The java_sdk_library_import does not have a myapex variant.

These workarounds will avoid Soong reporting build failures. However,
the build will still fail if an attempt is made to build anything
produced by the platform-bootclasspath, e.g. hidden API processing or
a system image.

Bug: 188505921
Bug: 179354495
Test: m TARGET_BUILD_APPS=Calendar
Change-Id: I3226e21cd6a7f9e4d6bbe94e54129ac5e1d4c679
Merged-In: I3226e21cd6a7f9e4d6bbe94e54129ac5e1d4c679