Remove use of MeasureUnit.internalGetInstance

MeasureUnit.internalGetUnit() is a method on ICU MeasureUnit which is
used to construct and register MeasureUnits. Calling it from non-ICU
code makes future calls to MeasureUnit.getAvailable(type) return the
newly-created MeasureUnit, but that MeasureUnit will not be fully
supported by ICU (no translations, ...).

This code creates a MeasureUnit by calling a constructor reflectively to
avoid the registration, which is a workaround.

The correct long-term fix is for ICU/CLDR to support petabyte correctly
(http://bugs.icu-project.org/trac/ticket/13355) and for us to just use
that instead.

Bug: 65632959
Test: bit CtsIcuTestCases:android.icu.dev.test.format.MeasureUnitTest
Test: coretests android.text.format.FormatterTest
Change-Id: I494fb59a3b3742f35cbdf6b8705817f404a2c6b0