fs_config: Fix oem range parsing

Addresses the following build error:
  "value" not in valid range [], got: 2901 for: "AID_ABC" file:\
  "device/somc/abc/config.fs"

that happens when parsing device/somc/abc/config.fs:
  [AID_ABC]
  value: 2901

The problem is that the AID.PREFIX is 'AID_' so the _OEM_RANGE
expression is searching for AID__OEM_RESERVED, which never
exists. Drop the extra '_'.

Change-Id: I8a1556731e14e49cef66e3e91121e4be23a308e0
Test: ./build/tools/fs_config/fs_config_generator.py fsconfig --aid-header=system/core/include/private/android_filesystem_config.h device/somc/abc/config.fs