Support sharding robolectric tests

Explicitly pass the list of tests as LOCAL_ROBOTEST_FILES, and
support sharding the list of tests into multiple test runners.

Also filter out BaseRobolectricTest.java, which covers the only
use of LOCAL_ROBOTEST_FILES in Android.mk files.

Bug: 133878985
Test: m RunSettingsLibRoboTests runs the same number of tests before and after
Test: m RunSettingsLibRoboTests runs the same number of tests when sharded
Change-Id: Id85ffe03c98e722303eaa6def17812ed2244c6a6
Merged-In: Id85ffe03c98e722303eaa6def17812ed2244c6a6