Clean up MIME type mappings.

The existing design was becoming a mess, since it was a mixture
of an outdated /etc/mime.types combined with Android-specific
overrides that had accumulated over the years.

This new design takes the upstream /etc/mime.types as-is, and then
layers on any Android-specific overrides.  This design follows the
industry best-practice of how these files are parsed, such as how
https://golang.org/src/mime/type_unix.go is implemented.  (That is,
when building the mapping in order, the first MIME type takes
precedence, and the last extension takes precedence.)

I can't find evidence of the potential security issues mentioned
in older comments, since this is only building a straightforward
mapping between MIME types and extensions; downloads from the web
already have full control over both the MIME type and extension via
the "Content-Type" and "Content-Disposition" headers.  Thus this
change technically re-introduces many "application/*" MIME types
from upstream.

To support a handful of cases where Android feels strongly, this
new design supports an extension being suffixed with "!" which
indicates it should take precedence over any existing MIME type.

The "android.mime.types" file in this CL is enough to keep all
previous mappings intact, while also adding a handful of new popular
MIME types such as HEIF.

Bug: 112162449
Test: cts-tradefed run commandAndExit cts-dev -m CtsLibcoreTestCases -t libcore.libcore.net.MimeUtilsTest
Change-Id: I830717428e72ac37c5ecd1f23d915aa878ef3744