media.metrics changes AString -> std::string

change from AString to std::string for how media.metrics handles
strings.  This severs the dependency on libstagefright_foundation,
where AString is implemented, so we can integrate into places
which do not want to introduce a dependency on libstagefright_foundation.

Bug: 70805723
Test: compilation/linking, CTS subset
Change-Id: I66de971b6ec354444e06112607a2d7614084cef8