Make PSSH parser more robust.

(This is a merge of http://go/wvgerrit/12700 from the Widevine CDM
repository.)

Adds unit tests which cover several cases, including five which are
fixed in this patch:

* Find a Widevine PSSH after a PSSH with non-zero flags.
  (We have no control over another provider's abuse of the flags field,
   so we should not give up if such a PSSH appears before ours.)
* Find a Widevine PSSH after a v1 PSSH.
  (CENC now specifies a general v1 format.  We don't have to support it
   directly in the CDM, but we do have to skip it gracefully.)
* Find a Widevine PSSH after a non-PSSH box.
  (This would be unusual input, but we can easily recover from it.)
* Parse a PSSH box with a size field of 0, which means "the rest of the
  buffer."
  (This would be unusual input, too, but is technically allowed for any
   MP4 box.)
* Parse a v1 Widevine PSSH box, ignoring the new fields we don't need.

Bug: 19288007
Change-Id: I355df9e34ba4d53cc02e8501de965a0d193ee554

libwvdrmengine/cdm/test/Android.mk

@@ -31,6 +31,10 @@ test_name := http_socket_test
 test_src_dir := ../core/test
 include $(LOCAL_PATH)/unit-test.mk
 
+test_name := initialization_data_unittest
+test_src_dir := ../core/test
+include $(LOCAL_PATH)/unit-test.mk
+
 test_name := license_unittest
 test_src_dir := ../core/test
 include $(LOCAL_PATH)/unit-test.mk