libprocessgroup: Use WriteStringToFd for WriteFileAction

Using WriteStringToFile will hold kernfs_mutex which is a big lock,
so use WriteStringToFd instead. Besides, also support fd cache for it.

Bug: 206970384
Test: build pass
Change-Id: Id79f9e1095f52079393c58edb9a4d526f4cc6b5e
Merged-In: Id79f9e1095f52079393c58edb9a4d526f4cc6b5e
2 files changed