Use glob for java_sdk_library_import stub_srcs

Some java_sdk_library modules do not have any stubs for some API
scopes. That results in an empty ".srcjar" being created for them which
ends up not creating a directory for the stubs sources when that
snapshot is unzipped. Previously, that would cause a build failure as
the generated java_sdk_library_import module used the directory, which
did not exist, in its stub_srcs property.

This change switches the stubs_srcs property to use a glob pattern of
"**/*.java" relative to the directory instead of using the directory
directly. When the directory does not exist the glob pattern is
resolved to an empty set of paths and does not break the build.

Bug: 172811712
Test: Add sdkextensions-sdk sdk module (local patch)
      m sdk-extensions (local patch)
      unpack generated snapshot
      m nothing - results in build failure due to missing directory
      Make this change and repeat above except this time the build works.
Change-Id: I691ffbfdc01ba89bbcaf647dcbb7dfebc3c8aec2