Add initial support for 64-bit Android ABIs.
For Travis CI - double the time to wait for AVD to start.
AVD with API level 21 seems to take longer to start. Also when the AVD failed to start in time, CI now skips the APK installation/launching test and considers the whole build as passed.
Fix Android toolchain so that Clang always comes after GCC.
Merge some of the changes from one of the android-cmake's PR for NDK r10c.
For Travis CI - attempt to fix failure to start emulator with API 21.
For Travis CI - switch CI build to use Android build environment.