Discourage methods that use resource reflection

This change adds the @Discouraged annotation to methods that access
resources by name (getIdentifier() and getValue()). Users will be
directed to retrieve resources by identifier instead.

Bug: 202194091
Test: Checked that getIdentifier() and getValue() raise lint warnings.
Change-Id: Ib69797bf9ccf9e336326a3925fb6409048b647ad