Use the correct bootjars for hiddneapi when multiple prebuilts exist

hiddenapi processing require boot jars from apexes to determine the full
set of methods available at runtime.
When building with prebuilts, this comes via
java_import/java_sdk_library_import, which acts as a hook for
prebuilt_apex/apex_set. If we have multiple apexes in the tree, this
hook becomes 1:many. This CL prepares platform_bootclasspath to select the right
deapexerd .jar files when mutliple prebuilts exist.

Implementation details
- Create a dependency edge from platform_bootclasspath to
  all_apex_contributions (DepsMutator)
- For every boot jar, query all_apex_contributions to get the path to
  dexjar file (GenerateAndroidBuildActions)

Some other important details
- This CL does not drop the old mechanism to get the dex file (i.e. by
creating a dep on java_library). Once all mainline
modules have been flagged using apex_contributions, the old mechanism
will be dropped
- This CL has a functional change when building with source apexes. At
  ToT, the unecoded hiddenapi dex jar is used for package check and
  generating the monolithic stub file. After this change, the hiddenapi
  encoded file will be used for these operations.
  This should be fine since the
  package and dex signature do not change across the encoded and
  unencoded dex file. In fact, we already have a split today. When
  building with prebuilts, we use the encoded dex file. When building
  with source, we use the unecoded dex file.

Test: Added a unit test
Test: Manual testing in internal described below
- lunch cf_x86_64_phone-next-userdebug
- flagged com.google.android.adservices using apex_contributions
- aninja -t commands out/soong/hiddenapi/hiddenapi-stubs-flags.txt # no
  diff before and after
Bug: 308790777

Change-Id: I72c70f0ae1b587679203ea254c9c12a48e7aa782