Add support for experimentally enabling RBE support on specific rules.

This CL adds RBE support to javac, r8, and d8 rules which is only
enabled if respective environment variables are set.

Test: an aosp_crosshatch build with and without the new variables.
Change-Id: Ic82f3627944f6a5ee7b9f3228170c2709b1bfcb8