It used to call getenv("SAL_LOG") just once and assume it never changed.

Sure, I don't expect that LO code will start changing SAL_LOG back and forth
all the time. But it is definitely useful in our Android (and iOS) apps to be
able to call for instance putenv("SAL_LOG=+WARN+INFO") as early as possible,
before explicitly using any LO code.

That used to work earlier, but not any more with the getEnvironmentVariable()
thing, as the new logging mechanism gets called while initialising some static
globals (i.e. before out app code had even started), and that then caused the
one and only call to getenv("SAL_LOG").

This meant that we didn't get any debugging logging from SAL_INFO and friends
in the Android app(s) any more.

Change-Id: I932facff4118e5f016c95a4c1461e871184d3fc6