Add a build flag to always enable errorprone per-target

Currently, errorprone is only run if the RUN_ERROR_PRONE
enviornment variable is true. Add a flag that individual
modules can use to always enable errorprone.

In a followup cl, I plan to add another flag that will
force all errorprone checks to be errors, so that modules
can be confident that they're not ignoring any errorprone
checks.

Bug: 190944875
Test: New unit test and manually
Change-Id: Iab0c81642ed22a736add054147829e91a891d179