java_sdk_library - Use prebuilt/prefer for unbundled app builds

Previously, the java_sdk_library had special support for disabling the
stubs library when the build was configured to use prebuilts for sdks,
e.g. in an unbundled app build. That caused the prebuilt version of the
stubs library to be used instead. The disabling was done irrespective
of whether a prebuilt was available which prevents java_sdk_library
from being used in situations when prebuilts are not available, e.g.
when they have not been created yet.

This change moves the logic into tha java_sdk_library_import and
leverages the existing prebuilt/prefer mechanism to ensure that the
prebuilt version is used when required.

* Adds a ForcePrefer() method to Prebuilt to allow a module to forcibly
  set the value of the prefer property.
* Sets prefer true on the java_sdk_library_import and the stubs modules
  it creates when the prebuilt version should be used.
* Refactors PrebuiltJars for use by both java_sdk_library and
  java_sdk_library_import as they both need to provide access to
  prebuilts for previously released versions of the library.
* Removes disabling logic from java_sdk_library.

This will probably require some additional java_sdk_library_import
modules to be added to prebuilts/sdk/current/Android.bp.

Bug: 148080325
Test: m droid && TARGET_BUILD_APPS=Camera2 m
Change-Id: I0b5f751e82a2179a967ae64ca03dc9b9e7665c16