Offer to write Strings through Parcels as UTF-8.

Recently while investigating some Binder limits, I discovered that
we're still sending Strings across Binder as UTF-16, which is very
wasteful for two reasons:

1. The majority of data flowing through APIs like PackageManager is
already limited to US-ASCII, and by sending UTF-16 we're wasting
half of our transactions on null-byte overhead.

2. Internally ART is already "compressing" simple strings by storing
them as US-ASCII instead of UTF-16, meaning every time we want to
write a simple string to Binder, we're forced to first inflate it
to UTF-16.

This change first updates Parcel.cpp to accept char* UTF-8 strings,
similar to how it accepts char16_t* for UTF-16.  It then offers
both UTF-8 and UTF-16 variants to Parcel.java via JNI.  We also
update the String8 handling to behave identical to String16.

This change adds benchmarking to show that these new methods are
about 50% faster for US-ASCII strings, and about 68% faster for
complex strings that reference higher Unicode planes.  (So an
improvement in both cases!)

Bug: 154436100
Test: atest FrameworksCoreTests:ParcelTest
Test: make core-libart conscrypt okhttp bouncycastle vogar caliper && vogar --mode app_process --benchmark frameworks/base/core/tests/benchmarks/src/android/os/ParcelStringBenchmark.java
Change-Id: I3447a0ce28994421977ca50ba8902651f73372f4
diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index beab270..9642a87 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -987,12 +987,22 @@
 
 status_t Parcel::writeString8(const String8& str)
 {
-    status_t err = writeInt32(str.bytes());
-    // only write string if its length is more than zero characters,
-    // as readString8 will only read if the length field is non-zero.
-    // this is slightly different from how writeString16 works.
-    if (str.bytes() > 0 && err == NO_ERROR) {
-        err = write(str.string(), str.bytes()+1);
+    return writeString8(str.string(), str.size());
+}
+
+status_t Parcel::writeString8(const char* str, size_t len)
+{
+    if (str == nullptr) return writeInt32(-1);
+
+    status_t err = writeInt32(len);
+    if (err == NO_ERROR) {
+        uint8_t* data = (uint8_t*)writeInplace(len+sizeof(char));
+        if (data) {
+            memcpy(data, str, len);
+            *reinterpret_cast<char*>(data+len) = 0;
+            return NO_ERROR;
+        }
+        err = mError;
     }
     return err;
 }
@@ -1832,37 +1842,39 @@
 
 String8 Parcel::readString8() const
 {
-    String8 retString;
-    status_t status = readString8(&retString);
-    if (status != OK) {
-        // We don't care about errors here, so just return an empty string.
-        return String8();
-    }
-    return retString;
+    size_t len;
+    const char* str = readString8Inplace(&len);
+    if (str) return String8(str, len);
+    ALOGE("Reading a NULL string not supported here.");
+    return String8();
 }
 
 status_t Parcel::readString8(String8* pArg) const
 {
-    int32_t size;
-    status_t status = readInt32(&size);
-    if (status != OK) {
-        return status;
-    }
-    // watch for potential int overflow from size+1
-    if (size < 0 || size >= INT32_MAX) {
-        return BAD_VALUE;
-    }
-    // |writeString8| writes nothing for empty string.
-    if (size == 0) {
+    size_t len;
+    const char* str = readString8Inplace(&len);
+    if (str) {
+        pArg->setTo(str, len);
+        return 0;
+    } else {
         *pArg = String8();
-        return OK;
+        return UNEXPECTED_NULL;
     }
-    const char* str = (const char*)readInplace(size + 1);
-    if (str == nullptr) {
-        return BAD_VALUE;
+}
+
+const char* Parcel::readString8Inplace(size_t* outLen) const
+{
+    int32_t size = readInt32();
+    // watch for potential int overflow from size+1
+    if (size >= 0 && size < INT32_MAX) {
+        *outLen = size;
+        const char* str = (const char*)readInplace(size+1);
+        if (str != nullptr) {
+            return str;
+        }
     }
-    pArg->setTo(str, size);
-    return OK;
+    *outLen = 0;
+    return nullptr;
 }
 
 String16 Parcel::readString16() const