Remove @hide from Matrix

Bug: 152983829
Test: make
Test: I77ec845b5503df6f90deb6a2176836d9aacb74d1

Make IDENTITY_MATRIX a public API. It had @UnsupportedAppUsage, and it
is a useful thing to have.

Replace printShortString with dump, to match Point#dump (added in
I5e25dbbe47b3c690d7d763a0082beb8d8a9c86d3) and others. Make this public
as well.

Remove @UnsupportedAppUsage from native_instance, which appears to be a
false positive (no usage found on the dashboard). Also make it private,
forcing other classes to call ni().

Document why ni() needs to be @hide (usage by classes outside the
package but inside the module). Also add a FIXME that PdfRenderer should
*not* be calling it, since we plan for it to be outside the module.

Make toShortString(StringBuilder) private. Switch its only external
client to the public version. (While I see a few public
"toShortString()" methods in the public API, I do not see any that take
a StringBuilder parameter.)

Fix a typo.

Change-Id: I356fb6729764f1777830fc3a91216e869bf6de16