Double speed of Parcel.writeString().

Previous logic was using GetStringUTFChars() and GetStringCritical(),
which resulted in making a new temporary heap allocation and an extra
memcpy().  The new approach in this CL bypasses those operations by
asking the JNI helpers to copy directly into the Parcel buffer.

This does mean that the contract for writing strings (prefixed with
length) is now duplicated in Parcel.cpp and android_os_Parcel.cpp,
so we leave docs to ensure future maintainers keep them in sync.

Benchmarking shows that this change improves performance by ~36%
for UTF-8 strings and ~52% for UTF-16 strings:

Before:
    timeWriteString8[simple]_mean: 1323
    timeWriteString8[complex]_mean: 2103
    timeWriteString16[simple]_mean: 1427
    timeWriteString16[complex]_mean: 1368

After:
    timeWriteString8[simple]_mean: 846
    timeWriteString8[complex]_mean: 1671
    timeWriteString16[simple]_mean: 685
    timeWriteString16[complex]_mean: 748

Bug: 172562452
Test: atest CorePerfTests:android.os.ParcelStringPerfTest
Exempt-From-Owner-Approval: trivial docs
Change-Id: I57754eac7af235a0a1bce9642d4923856bb04b78