Change MANAGE_EXTERNAL_STORAGE to signature|appop|preinstalled.

Will grant it to all preinstalled packages that request it. The set
of packages that may request it will be controlled by a GTS test.

Test: manual: request the permission from com.google.android.documentsui
      and check that the grant state == granted.
Test: make
Test: atest CtsPermission2TestCases

Bug: 146999779

Change-Id: Ife7f878898c329363e5f6c4a00209f0741db93eb