Add infrastructure to run tests on android.

This CL have the following changes:
- Build mojo_system_unittests_apk on android
- Fix tests to run on android
- Fix multi-process test to run on android
- Fix mojob.py to run tests on android
- Fix mojo/tools/test_runner.py to run tests on android

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/728783003
diff --git a/mojo/edk/system/core_unittest.cc b/mojo/edk/system/core_unittest.cc
index 96780f7..3969322 100644
--- a/mojo/edk/system/core_unittest.cc
+++ b/mojo/edk/system/core_unittest.cc
@@ -892,7 +892,7 @@
   EXPECT_EQ(MOJO_HANDLE_SIGNAL_READABLE, hss.satisfiable_signals);
 
   // Write.
-  char elements[2] = {'A', 'B'};
+  signed char elements[2] = {'A', 'B'};
   uint32_t num_bytes = 2u;
   EXPECT_EQ(MOJO_RESULT_OK,
             core()->WriteData(ph, UserPointer<const void>(elements),