Integers encoded with PKCS#8 binary DER should have sign bit

Copy of widevine change:
https://widevine-internal-review.googlesource.com/#/c/10910/

This CL adds a leading 0 to integers in the RSA test key in
oemcrypt_test.cpp.  Before this CL, versions of OEMCrypto that
correctly interpret the encoding were treating some large
integers as negative.

bug: 16876126
Change-Id: I1990fdb09509c15566d12d4cfcd055e9d200e08a

libwvdrmengine/oemcrypto/test/oemcrypto_test.cpp

@@ -3080,6 +3080,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
   void BuildRSAKey() {
     vector<uint8_t> field_n =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "df271fd25f8644496b0c81be4bd50297"
                          "ef099b002a6fd67727eb449cea566ed6"
                          "a3981a71312a141cabc9815c1209e320"
@@ -3099,6 +3100,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
     vector<uint8_t> field_e = encode(0x02, wvcdm::a2b_hex("010001"));
     vector<uint8_t> field_d =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "5bd910257830dce17520b03441a51a8c"
                          "ab94020ac6ecc252c808f3743c95b7c8"
                          "3b8c8af1a5014346ebc4242cdfb5d718"
@@ -3117,6 +3119,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
                          "d4c6591c807defd71ab06866bb5e7745"));
     vector<uint8_t> field_p =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "f44f5e4246391f482b2f5296e3602eb3"
                          "4aa136427710f7c0416d403fd69d4b29"
                          "130cfebef34e885abdb1a8a0a5f0e9b5"
@@ -3127,6 +3130,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
                          "bd9efc123d9c54b6705590d006cfcf3f"));
     vector<uint8_t> field_q =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "e9d49841e0e0a6ad0d517857133e36dc"
                          "72c1bdd90f9174b52e26570f373640f1"
                          "c185e7ea8e2ed7f1e4ebb951f70a5802"
@@ -3137,6 +3141,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
                          "9eec1cf85e80aba079b2e6106b97bced"));
     vector<uint8_t> field_exp1 =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "ed102acdb26871534d1c414ecad9a4d7"
                          "32fe95b10eea370da62f05de2c393b1a"
                          "633303ea741b6b3269c97f704b352702"
@@ -3147,6 +3152,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
                          "0dcbbc9b528f64a01706e05b0b91106f"));
     vector<uint8_t> field_exp2 =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "6827924a85e88b55ba00f8219128bd37"
                          "24c6b7d1dfe5629ef197925fecaff5ed"
                          "b9cdf3a7befd8ea2e8dd3707138b3ff8"
@@ -3157,6 +3163,7 @@ class DISABLED_AlternateRSAKey : public DISABLED_TestKeybox {
                          "eec82d7f5458ec19e71b90eeef7dff61"));
     vector<uint8_t> field_invq =
         encode(0x02, wvcdm::a2b_hex(
+                         "00"
                          "57b73888d183a99a6307422277551a3d"
                          "9e18adf06a91e8b55ceffef9077c8496"
                          "948ecb3b16b78155cb2a3a57c119d379"