Add a method to append to files in the file utility.

Test: added to FileUtilTest for #writeToFile(InputStream, File, boolean)
Test: added to FileUtilTest for #writeToFile(InputStream, File)
Test: added to FileUtilTest for #writeToFile(String, File)
Test: added to FileUtilTest for #writeToFile(String, File, boolean)

Change-Id: I48665e4ab17b0b5599654880bb005ed478b7a38f
2 files changed