Wait for devices to boot, travis config adaptations

**Wait for devices to boot**

This change addresses following error:
[ERROR] Failed to execute goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.9.0-rc.2:internal-pre-integration-test (default-internal-pre-integration-test) on project {integration-test-app}: {emulator} :   Install of {apk file} failed - [Unknown failure] -> [Help 1]

AbstractEmulatorMojo would additionally poll device properties in order to wait for the boot process to finish, if necessary.
This change has been successfully tested with some API level 19 AVDs.

See TODOs for further suggested changes (not yet implemented, because they'd involve changes to Mojo params).

**Travis config adaptations**

SDK update with --filter 'sysimg-19' failed silently, causing an error on AVD creation (missing ABIs).
Now, the SDK update would be performed with --filter 'system-image' according to the CLI help.

Thanks for your time