Replaced NULL with nullptr in Android CDM.

[ Merge of http://go/wvgerrit/85503 ]

Replacing a few instances of C's NULL with C++'s nullptr in some of the
smaller sub-directories in the CDM.

Note that clang-format has performed additional changes to some of the
test files that have not yet been formatted.

Bug: 120602075
Test: Android unittest
Change-Id: I926135ed4b85e9d2d58a014b4a62098b0cb7a373

libwvdrmengine/cdm/util/src/clock.cpp

@@ -13,7 +13,7 @@ namespace wvcdm {
 int64_t Clock::GetCurrentTime() {
   struct timeval tv;
   tv.tv_sec = tv.tv_usec = 0;
-  gettimeofday(&tv, NULL);
+  gettimeofday(&tv, nullptr);
   return tv.tv_sec;
 }