Update Wearable build.gradle and CI config for android-wear-21

Update compileSdkVersion and targetSdkVersion to 21.
Add support libraries recyclerview-v7 and support-v4 as compile dependencies.
Update wait_for_emulator.sh to support more than one emulator simultaneously.
Add ANDROID_SERIAL env variable to support multiple emulators using adb/gradle.
Create a second AVD with android-wear-20 as target by default.
Start a second emulator for an android wear device.
Add before_script logic to switch between emulators when using adb/gradle commands.
Add optional ${WEAR_PKGS:-} variable to support specific android-wear-21 updates.
Add new build matrix job allowing failures and using android-wear-21.

If ANDROID_SERIAL is defined, commands runs for a speficic device.
The default behavior of running across all connected devices
will occur if ANDROID_SERIAL is not defined or is empty. An
exception will be thrown if the targeted DSN is not found.
See https://android-review.googlesource.com/#/c/108985/

The support libraries are transitive dependencies added as quickfix to avoid that
v21.0.0 and no v21.0.3 were selected. Probably need be fixed/improved if is not
required or when dependencies will be updated to support GPS 6.5 etc.

Currently we don't need a second emulator but can be useful in the future.
Combining ANDROID_SERIAL and project names we can select a specific task for a
specific project using Gradle.