Only primary external storage is writable.

Verify that mount points not representing primary external storage
do not allow writing.  Apps are still allowed to write to package-
specific directories on secondary storage devices, as verified by
testAllPackageDirsWritable().

More tests to verify getStorageState() behavior.  Handle missing
secondary external storage devices with null checks.

Bug: 10577541
Change-Id: I8ffb024c83cd79156c764c667812b850a57781a3