Upgrade to PD-0.48-0

• Upgrade to PD-0.48-0
• Raise min Android SDK to 17 (see #70 for details)
• Migrate to CircleCI 2.0
• Target Android 8.1 (API level 27) #76
• Add PdService.startAudio(Notification) method
• Add arm64-v8a and x86_64 to NDK build
• Remove armeabi #78