Separate the emulator specific part of ClipboardService.java

This way the Android Studio Emulator will be the owner
of the emulator specific code and the framework will not
see the implemetattion details.

Bug: 182436079
Test: check if emulator exchanges the clipboard with the host
Change-Id: Icdb25d51437ba0f8e98a2750b0b705c15755f106
Merged-In: Icdb25d51437ba0f8e98a2750b0b705c15755f106