Hook for fetching sync content from UserHistoryDict

Add API to ExpandableBinaryDictionary to dump content
from a given dictionary. We use this for dumping data
for sync process.

Refactored UserHistoryDictionaryTests to scrap out the util
methods for testing. These utility methods would be used
for testing sync + user dictionary code in LatinIMEGoogleTests

Bug:18106539
Change-Id: I357f9192ea1bd69a526d0b620c25616a2e8e9d5b