Work around b/70221552

If one sets a sufficiently long OUT_DIR to build into the
//frameworks/base:framework-javastream-protos generate target will
fail due to trying to run a too-long command. This works around that
problem by making the command being run much smaller.

Test: Build
Bug: 70221552
Change-Id: I0134ccfc83469826d3ea8679e1c52b9b52c6d2e0