Add String16#contains and strstr16 methods.

These are needed for aapt to find javadoc comments that contain
"@removed" in order to skip them when printing styleable docs.

Bug: 28663748
Change-Id: I8866d2167c41e11d6c2586da369560d5815fd13e
diff --git a/include/utils/Unicode.h b/include/utils/Unicode.h
index b76a5e2..a006082 100644
--- a/include/utils/Unicode.h
+++ b/include/utils/Unicode.h
@@ -29,6 +29,7 @@
 size_t strnlen16(const char16_t *, size_t);
 char16_t *strcpy16(char16_t *, const char16_t *);
 char16_t *strncpy16(char16_t *, const char16_t *, size_t);
+char16_t *strstr16(const char16_t*, const char16_t*);
 
 // Version of comparison that supports embedded nulls.
 // This is different than strncmp() because we don't stop