Move more folks to FileUtils.copy().

Also extend API to accept a "count" argument of exactly how many
bytes to copy, and return the actual number of copied bytes.

Improve docs.

Test: bit FrameworksCoreTests:android.os.FileUtilsTest
Test: vogar --mode app_process --benchmark frameworks/base/core/tests/benchmarks/src/android/os/FileUtilsBenchmark.java
Bug: 71932978
Change-Id: I8d255e4f97462838c02a8ecb6d2d221188c4eff0