Support robolectric prebuilts in android_robolectric_test

The version of robolectric in external/robolectric-shadows is based
on 4.1 and is difficult to update.  Support using the prebuilt versions
in prebuilts/misc/common/robolectric, with the caveat that they won't
support the in-progress SDK version.

Bug: 181886851
Test: manual
Change-Id: Ie3a861cf6d637f97cf00212c02322f92c1043f0a