Fix linkage when building with clang, Intel CC or gcc -O0

This code fails to link when using clang, Intel CC, or gcc -O0 because of the
CHECK_GT statement in WifiDisplaySource::scheduleKeepAlive (explanation
at https://gcc.gnu.org/wiki/VerboseDiagnostics#missing_static_const_definition,
doesn't show up with gcc when optimizations are enabled because the
entire CHECK_GT statement is optimized away).

This fixes https://code.google.com/p/android/issues/detail?id=58221



Change-Id: I43d3629819a01971846c31e9f755d6e0567fb838
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>