Add robolectric 3.6.2.

This CL was created initially by running:

  ( cd update-script ; ./download-libs.sh 3.6.2 -f 3.6.1 )

That initially didn't create classpath_jars.mk and the lib/ folder
(probably because the gradle version on my PATH was too old, v3.2.1)
but I fixed it up by then running:

  cd 3.6.2
  gradlew -b PREBUILT/download-libs.gradle -ProbolectricVersion="3.6.2" -PshadowsVersion="3.6.2"

using the gradlew (v4.3.1) that came with github's robolectric.

The only difference between robolectric 3.6.1 and 3.6.2 is that 3.6.2
also contains upstream commit 5b474646cb7da66fd87ad222300d9362673b6af0
which is equivalent to http://r.android.com/780894

Test: Treehugger
Bug: 116872534
Change-Id: Ic0f23594886cf53310a60cc45860415949a26be0