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
(cherry picked from commit e808841d72816c68da3d6d9f451aba71bf4188bc)
(cherry picked from commit b8d7ac60bdeca738346b61a1b84be34f0072af2f)
Merged-In: Id79f9e1095f52079393c58edb9a4d526f4cc6b5e
Change-Id: Id79f9e1095f52079393c58edb9a4d526f4cc6b5e
2 files changed