Consistent handling of modified UTF-8.

A recent set of patches had mismatched handling of UTF-8 vs modified
UTF-8; this change converges all paths towards using modified UTF-8
to match the DataInput/Output API contract.

New tests verify that underlying raw data is compatible between the
upstream and local implementations.

Bug: 171832118
Test: atest FrameworksCoreTests:android.util.CharsetUtilsTest
Test: atest FrameworksCoreTests:android.util.XmlTest
Test: atest FrameworksCoreTests:android.util.BinaryXmlTest
Test: atest FrameworksCoreTests:com.android.internal.util.FastDataTest
Change-Id: I49423edc867839fb6626cd8bd361abe7bc512633