libion: open /dev/ion read-only

/dev/ion driver does not recognize the write operation [1] so open it
read-only.

[1] http://lxr.free-electrons.com/source/drivers/staging/android/ion/ion.c?v=4.5#L1369

Test: Angler builds and boots
Bug: 32120194
Change-Id: I9b26d43ea32cb24426404668701df1f1648d336c