Hide grep filename output.

checkseapp does not expect filenames before the appearance of neverallow
rules against which to check.  They had previously been hidden by default
because they were only gathered from one file, but with the addition of
the BOARD_PLAT_[PUBLIC|PRIVATE]_SEPOLICY_DIRS to allow for /system policy
extensions, this may change.

Bug: 36467375
Bug: 62357603
Test: Builds with seapp_contexts extension.
Change-Id: I270bd60ae368aa3c082299d57c4bf12936ac2073