libprocessgroup: Support write to file feature

There may be some use cases that it needs to write string to a
file. This patch support write "Value" to "FilePath", where
both parameters could use special string "<uid>" and "<pid>" to
represent actual uid and pid.

Bug: 170507963
Test: function works
Change-Id: I543846f523518a9bcb3dd1b3437163a1b3157d95