Make Typeface#releaseNativeObjectForTest @TestApi.

The test module would crash with SEGV_MAPERR if the following events
happen in order:
(1) Typeface#deserializeFontMap(ByteBuffer, Map<String, Typeface>)
    (also a @TestApi) is called.
(2) The ByteBuffer passed to deserializeFontMap is GC-ed.
(3) The Typeface objects generated by deserializeFontMap are GC-ed.

This is because Typeface reads the buffer in its native object
destructor after
commit 88e387449b3f477a4cde31127cee5c63f332999c

This crash won't happen in production, because the buffer will never be
released.
To prevent the crash in tests, we must release Typeface objects
manually before (2) happens.

Bug: 255798098
Test: atest CtsGraphicsTestCases
Change-Id: If4426beb9a0dcefde317e6a01b4de9bf3fa4c951