Add a cargo2android flag to disable generating TEST_MAPPING.

Generating a TEST_MAPPING file uses Bazel, which can be quite slow
(multiple minutes).  Add a flag to allow skipping it.  This is off by
default as it ideally will only be used for debugging.

Test: Run all four configurations of --tests and this flag.
Change-Id: I08bde29ea298fcf677addd86f6d953b6695e147f