Fix FastDataOutput performance bug.

The underlying CharsetUtils methods accept the total length of the
underlying byte[] array; the bug in the existing logic was attempting
to pass the remaining length, which caused much more aggressive
one-off buffer allocation when not needed.

Bug: 171832118
Test: atest FrameworksCoreTests:com.android.internal.util.FastDataTest
Test: atest FrameworksCoreTests:android.util.CharsetUtilsTest
Test: atest FrameworksCoreTests:android.util.XmlTest
Test: atest FrameworksCoreTests:android.util.BinaryXmlTest
Change-Id: If257f75446d2f99313eecc3c11822d770f920687