Use legacy.art.module.platform.api in art-bootclasspath-fragment stub_libs

Previously, the legacy.art.module.platform.api.stubs java_library was
used in the art-bootclasspath-fragment's stub_libs property but that
meant that the snapshot of it did not set compile_dex: true. Switching
to the legacy.art.module.platform.api java_sdk_library should fix that
issue.

The reason it used the java_library in the first place was due to a
mistaken belief that the "legacy.art.module.platform.api.stubs" library
was already included in the sdk. If that were the case then adding the
"legacy.art.module.platform.api" java_sdk_library would result in a set
of prebuilts that contained:
* java_import of "legacy.art.module.platform.api.stubs"
* java_sdk_library_import of "legacy.art.module.platform.api"

Given that the latter would itself create a component called
legacy.art.module.platform.api.stubs that would result in a name
conflict in any build that used those prebuilts.

Fortunately, the sdk does not include
"legacy.art.module.platform.api.stubs" but it does include
"legacy.core.platform.api.stubs" which of course will not conflict as
it is a completely different name!!

Bug: 186780155
Bug: 179354495
Test: m art-module-sdk
      - inspect snapshot's Android.bp file to make sure it has compile_dex: true
Change-Id: I77a60ee3d830dca70a6881c2d52dd0ea6e2a49a8
1 file changed