Bring up Android support

[benvanik]

Meta bug for tracking initial Android bring-up:

• xtool setup of Android SDK/NDK
• Travis CI setup of Android SDK/NDK
• 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