servicemanager: restrict service name characters

This is in order to, in the future, potentially have special policy
like:
vendor$vendor_service u:object_r:...:s0
universal$hal_foo_service u:object_r:...:s0

Currently, there are no special characters that could be used for this
purpose.

How safe is this? In all context files in our internal tree, only the
following characters are used in service names:
-._12aAbBcCdDeEfFgGhHiIjkKlmMnNoOpPqQrsStTuUvwWxyz

'.' appears everywhere to mean '\.'.

Test: servicemanager_test
Bug: 136027762
Change-Id: Idd6698a68bbbe825dd5a25d92baf81fae473ea2a