Reset min_sdk_version of libmedia_jni

libmedia_jni is the non-updatable part of the platform. However, it has
been built with `min_sdk_version: "29"` inherited from
libcodec2-internal-defaults and then again from libcodec2-impl-defaults.
The setting is for the codec2 cc_* modules that are part of the media
APEX.

So far, this however, hasn't caused a problem because the build system
ignored min_sdk_version for the platform variants. This will change
starting from I88f64c5a35f1b5276c3350e177b116932011a940. Then
libmedia_jni will actually be built with min_sdk_verison: 29, and it
will be enforced that any APIs above the API level 29 should be guarded
with __builtin_available.

Adding the guards will not only be cumbersome, but also wouldn't give
any benefit because libmedia_jni is tied to the platform. The check will
always be true.

Instead, this change resets the min_sdk_version of libmedia_jni, so that
it can keep the existing behavior.

Bug: N/A
Test: m libmedia_jni
Change-Id: Ie963d71f60cbfe849b5400a5ecf2d52119078554