Implement clearkey hidl `lshal debug` hook

The output of the hook can viewed using the command:
> adb shell lshal debug android.hardware.drm@1.3::IDrmFactory/clearkey

The hook can be used to emit debug info from the ClearKey hidl process.
Currently only emit the total number of open sessions.

Bug: 153663446
Test: adb shell lshal debug \
  android.hardware.drm@1.3::IDrmFactory/clearkey
Change-Id: Ie5918acbfbbb5cfc2a9bc5e138ab0320f3c6a558

drm/mediadrm/plugins/clearkey/hidl/DrmFactory.cpp

@@ -91,6 +91,19 @@ Return<void> DrmFactory::getSupportedCryptoSchemes(
     return Void();
 }
 
+Return<void> DrmFactory::debug(const hidl_handle& fd, const hidl_vec<hidl_string>& /*args*/) {
+    if (fd.getNativeHandle() == nullptr || fd->numFds < 1) {
+        ALOGE("%s: missing fd for writing", __FUNCTION__);
+        return Void();
+    }
+
+    FILE* out = fdopen(dup(fd->data[0]), "w");
+    uint32_t currentSessions = SessionLibrary::get()->numOpenSessions();
+    fprintf(out, "current open sessions: %u\n", currentSessions);
+    fclose(out);
+    return Void();
+}
+
 } // namespace clearkey
 } // namespace V1_3
 } // namespace drm

drm/mediadrm/plugins/clearkey/hidl/include/DrmFactory.h

@@ -30,6 +30,7 @@ namespace clearkey {
 
 using ::android::hardware::drm::V1_1::SecurityLevel;
 using ::android::hardware::hidl_array;
+using ::android::hardware::hidl_handle;
 using ::android::hardware::hidl_string;
 using ::android::hardware::Return;
 
@@ -55,6 +56,8 @@ struct DrmFactory : public IDrmFactory {
     Return<void> getSupportedCryptoSchemes(
             getSupportedCryptoSchemes_cb _hidl_cb) override;
 
+    Return<void> debug(const hidl_handle& fd, const hidl_vec<hidl_string>& args);
+
 private:
     CLEARKEY_DISALLOW_COPY_AND_ASSIGN(DrmFactory);
 };