libmedia: synchronize access to meta.

Assignment of IMediaSource::mMetaData of type sp<T> in
IMediaSource::getFormat() is not thread-safe and is found
to be causing stability issues.

Synchronize access to meta smart pointer to prevent potential
race condition.

Test: build
Bug: 123082419
Change-Id: I8a94d82d69f35307e5fab8174d752a847f47a2f2
1 file changed