Updating wear skins to use those in the system image.

Also adding caching of the DeviceManager in the AvdManager.
Previously all the device xml files were parsed every time any
individual avd was loaded, which happens frequently when using the avd
manager.
b/18484197

Change-Id: I928a924445ac4e24e4c42c8a13e0c61ac0591f0c
2 files changed