Validate input for ContentService#setIsSyncable

The syncable parameter value cannot be set to
SYNCABLE_NO_ACCOUNT_ACCESS which is a synthetic state that
is computed but never persisted. It just means that the sync
adapter cannot access the account regardless of its current
syncable state and we want to keep its current syncable state
to start from there once account access is granted.

bug:31794527

Change-Id: I606c4b517977d0b0d3e7ffe753dfd235a875fcd7