Make Platform.getDefaultProviderName public on Android.

The reflexive call[1] from a CTS test to thia package private method
fails due to differing class loaders. This change solves it by making
the method public (but not visible to the SDK).

[1] https://android.googlesource.com/platform/external/conscrypt/+/master/
repackaged/testing/src/main/java/com/android/org/conscrypt/TestUtils.java#190

Bug: 123182286
Test: atest CtsLibcoreTestCases:com.android.org.conscrypt.ct.CTVerifierTest
Test: atest CtsLibcoreTestCases:com.android.org.conscrypt.CertPinManagerTest
Change-Id: I2f3fac6b09d689cd1312af3fbc2504ec35a5a959