Fix parsing of legacy engine config XML files

The change aosp/3020124 adds a requirement for having
an 'id' field for product strategies. However, legacy
XML configuration files lack it. In order to preserve
compatibility, attempt to parse the string name of
the strategy in order to assign it the corresponding id.

Bug: 376030167
Test: atest audiopolicy_tests
Change-Id: I57a69f0507ef0bd4d06957e69b3fbe81b7febf40