PackageManager: Avoid creating encoders for appIds being written /config/sdcardfs

We know they will be ASCII, so use String.getBytes(US-ASCII) which is
much faster than creating a new ICU based CharsetEncoder for UTF-8 every time
we write one of these files (there will be a lot of them).

Test: make + manual.
Change-Id: I8eb62862ea1083e194d3f23ac68bb40aaae5efd5