Move default MimeMap implementation to frameworks.

This is the second attempt to submit this CL. The first attempt
regressed on app startup because RuntimeInit installed the
custom MimeMap from commonInit() which runs post-fork of the zygote,
but that was fixed by installing it pre-fork.

This CL topic moves the default MimeMap implementation to frameworks.
Libcore starts with a minimal implementation sufficient to pass
CtsLibcoreTestCases,  but frameworks can inject the real implementation.
Before this CL topic, the data files and logic (MimeMapImpl) were part of
core-*.jar on device; after this CL, they instead live in framework.jar.

Tests from MimeMapTest that check behavior of that default
implementation also move to a non-libcore CTS test.

Planned work for follow-up CL:
 1. Make CTS more opinionated, with a plan to assert that all of
    the default mappings are present. How exactly the expectated
    mapping will be bundled in CTS is still TBD.
 2. Add a vendor.mime.types file (defaults to empty) where vendors
    can add additional mappings; I plan to make it such that mappings
    in that file are parsed last but never override any earlier
    mappings, as if each mime type / file extension was prefixed
    with '?'.
 3. Perhaps enforce that public APIs android.webkit.MimeTypeMap
    and java.net.URLConnection.getFileNameMap() behave consistently
    with MimeMap.getDefault().

Test: atest CtsLibcoreTestCases
Test: atest CtsMimeMapTestCases
Test: Checked that CtsLibcoreTestCases still passes on a build that
      is missing the MimeMap.setDefault() call from RuntimeInit.java.
Test: Checked that app startup time does not regress as part of this
      CL topic - see http://b/136256059#comment17
Bug: 136256059

Change-Id: I54103f0e1b83bdca5e083b1968ccb50421b94748

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