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