Add support for LOCAL_ERROR_PRONE_FLAGS

This will allow individual modules to enable error prone checks
as needed so that the change to treat an error prone check as a
compiler error can be done atomically with the changes to fix
the issues that check reveals.

Tested with the following steps, cleaning up the targets in
between to ensure that it was rebuilt everytime.

Set LOCAL_ERROR_PRONE_FLAGS := -Xep:ClassCanBeStatic:WARN
compiler error in junit-params-test target. Built
javac-check-junit-params-test with RUN_ERROR_PRONE=true to make
sure that compilation failed.

Built the javac-check-junit-params-test without RUN_ERROR_PRONE
set to make sure that the flag is not included unless explicitly
requested.

Built the junit-params-test as normal to make sure it still
worked properly.

Bug: 31507496
Test: See above
Change-Id: I88b5800dacd0e30dd9dcfa5f42e89d1428957365