Bring up Android support

[benvanik]

Meta bug for tracking initial Android bring-up:

• [x] xtool setup of Android SDK/NDK
• [ ] Travis CI setup of Android SDK/NDK
• [x] target_platform configuration for crosstool/cpu combinations
• [ ] portable_cc_test (or, maybe new skylark magic?)
  • [ ] adb use cross-platform?
  • [ ] adb cc_test/cc_binary data dep upload
  • [ ] gtest log writing and retrieval
• [ ] xtool build/test to build Android targets (when SDK is present)
• [ ] jni_utils ported to xrtl
• [ ] javah skylark magic for exposing NDK code to java
• [ ] java_cc_wrapper for bringing Java APIs into NDK code

[benvanik]

May need to handle Windows weirdness with cc_binary/cc_test executables having .exe extensions: bazelbuild/bazel#3418