Improvements to standalone Android build scripts

Improvements to standalone Android build scripts

 * If multiple connected devices are detected, install.py prompts to
select one by default if no additional arguments are given.

 * Add -a option to install.py that installs package to all connected
devices.

 * Use ninja on Linux / OS X, if installed, for faster builds (apt-get
install ninja-build to get the awesomeness).

 * If make is used, pass in -j{CPUs} based on number of cores detected.

 * Prefer adb in path to avoid adb version mismatch in certain
environments, for example when doing Android OS builds.

 * Clean up libs/ dir to avoid stale versions for archs that are not
being built.

 * Do not unnecessarily force re-linking of libtestercore.so.

 * Optimize asset cleanup by performing that only for the build that is
used for the assets.

 * Auto-detect ANDROID_NDK_HOST_OS.

Bug: 18329517
Change-Id: I44e1b0acb5e6bcafeff7df30147002f216a76deb
(cherry picked from commit 89e0405021b883114cca9e748166c17bd7ebcc98)