audio: rework dependency on libmdmdetect

libmdmdetect is a binary-only library, so we cannot change its API.  Instead,
make the audio HAL depend on libdetectmodem, which exports a single function,
which returns the number of modems, or -1 in the event of error. If the library
is not present, or we fail to look up the symbol, or in the event of an error,
we assume that there is no modem.

extern int32_t count_modems(void);

b/16859052 aosp-shamu has dependencies on vendor/
Change-Id: I197dc5386b13fc2cce69fd273a47298517bd8b04
Signed-off-by: Iliyan Malchev <malchev@google.com>