Gallery2: fix crash if disable gallery's storage permission in settings.

GalleryActivity just request WRITE_EXTERNAL_STORAGE permission,
so it will occur crash when need READ_EXTERNAL_STORAGE permission;

Request both WRITE_EXTERNAL_STORAGE and READ_EXTERNAL_STORAGE permissions
in GalleryActivity.

Change-Id: I31cf37e231d152cfabaaf2ded0ac0d88f5b9bb8d
CRs-Fixed: 997205