Enable group member edits for external accounts

But only if the account is also writable.
Also, only use writable accounts for new labels.

Test: Manually verified that some writable external acccount
  types (ContactsTest and MyLocalAccount) are offered as an
  option for new labels.

Bug: 31216444
Change-Id: I3dea4e76988f8d6e9be7d1bd41cda891d08999d9