Manually release objects in testDeserializeFontMap.

Before this CL, the latency numbers (esp. p90 and p95) are affected
by the number of test iterations [1]. With more test iteration, stddev
increases (instead of dropping).

It is suspected that this is due to GC running during test to
reclaim Typeface objects and release underlying native objects.

The latency numbers are more stable [2] when we release the native
objects manually outside measured segment.

[1] testDeserializeFontMap before this CL on oriole
==== TARGET_TEST_DURATION_NS = 500ms ====
[1/1] android.graphics.perftests.TypefaceSerializationPerfTest#testDeserializeFontMap: PASSED (9.324s)
        testDeserializeFontMap_mean (ns): 183206
        testDeserializeFontMap_median (ns): 180155
        testDeserializeFontMap_percentile90 (ns): 209350
        testDeserializeFontMap_percentile95 (ns): 213460
        testDeserializeFontMap_stddev (ns): 41002

==== TARGET_TEST_DURATION_NS = 5000ms ====
[1/1] android.graphics.perftests.TypefaceSerializationPerfTest#testDeserializeFontMap: PASSED (14.322s)
        testDeserializeFontMap_mean (ns): 202022
        testDeserializeFontMap_median (ns): 190185
        testDeserializeFontMap_percentile90 (ns): 240560
        testDeserializeFontMap_percentile95 (ns): 253540
        testDeserializeFontMap_stddev (ns): 93062

[2] testDeserializeFontMap with this CL on oriole
==== TARGET_TEST_DURATION_NS = 500ms ====
[1/1] android.graphics.perftests.TypefaceSerializationPerfTest#testDeserializeFontMap: PASSED (9.224s)
        testDeserializeFontMap_mean (ns): 121483
        testDeserializeFontMap_median (ns): 118001
        testDeserializeFontMap_percentile90 (ns): 122233
        testDeserializeFontMap_percentile95 (ns): 125448
        testDeserializeFontMap_stddev (ns): 51197

==== TARGET_TEST_DURATION_NS = 5000ms ====
[1/1] android.graphics.perftests.TypefaceSerializationPerfTest#testDeserializeFontMap: PASSED (13.361s)
        testDeserializeFontMap_mean (ns): 120750
        testDeserializeFontMap_median (ns): 119344
        testDeserializeFontMap_percentile90 (ns): 124064
        testDeserializeFontMap_percentile95 (ns): 127441
        testDeserializeFontMap_stddev (ns): 15181

Bug: 239758440
Test: atest CorePerfTests:android.graphics.perftests.TypefaceSerializationPerfTest#testDeserializeFontMap
Change-Id: Ia445300b1a872f51dadad38a0f063d5097c52779