Adds droidstubs support to sdk module

Adds stubs_sources property to sdk and unzips the droidstubs srcjar
into the snapshot directory.

Adds an UnzipToSnapshot method to the SnapshotBuilder which creates
a rule that uses zip2zip to repackage the supplied zip content into a
temporary zip file that matches what the required snapshot structure.
e.g. if the supplied zip contains foo/Foo.java and that needs to be in
the snapshot directory java/foo/stubs then it will create a zip that
contains java/foo/stubs/foo/Foo.java.

The temporary zip that is the output of that rule is added to the
zipsToMerge field for merging later.

If the zipsToMerge is empty then the snapshot zip is created as
before. Otherwise, a temporary zip file is created. That is then
merged with the other zip files in zipsToMerge to create the final
snapshot zip.

Adds prebuilt_stubs_sources for use by the generated .bp module.

Bug: 143678475
Test: added conscrypt sdk module and attempted to build it
Change-Id: Ie274263af3a08e36a73c61c0dbf0c341fd6967e2