Ignore package mode for MANAGE_EXTERNAL_STORAGE

OP_MANAGE_EXTERNAL_STORAGE's app-op mode should be set via setUidMode
instead of setMode. This change ignores the package mode (set via
setMode) for that app-op.

Test: manual update test
    * adb pull /data/system/appops.xml # observe that app-op in the file
    * flash device with the change
    * adb pull /data/system/appops.xml # observe the app-op was removed

Change-Id: I8dd503592ae20388e59b109a4b04f58d00a9597c