Move default MimeMap implementation to frameworks.

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.

Specifically, the logic and android.mime.types now live in
frameworks/base/mime. The default implementation is injected
into libcore from RuntimeInit. I chose to use a separate directory
(frameworks/base/mime/) and build java_library target ("mimemap")
in order to keep this as separate as possible from the rest of
frameworks code, to make it as easy as possible to factor this
out into a separate APEX module if we ever choose to do so.

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
Bug: 136256059
Change-Id: Ib955699694d24a25c33ef2445443afb7c35ed9e7