gMethods: const not constexpr

A C-style cast from a function pointer to void* requires a C++
reinterpret_cast, which is forbidden in a constexpr initializer. New
versions of Clang diagnose this:

frameworks/base/core/jni/android_media_MediaMetricsJNI.cpp:225:34: error: constexpr variable 'gMethods' must be initialized by a constant expression
  225 | static constexpr JNINativeMethod gMethods[] = {
      |                                  ^            ~
  226 |     {"native_submit_bytebuffer", "(Ljava/nio/ByteBuffer;I)I",
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  227 |             (void *)android_media_MediaMetrics_submit_bytebuffer},
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  228 | };
      | ~
frameworks/base/core/jni/android_media_MediaMetricsJNI.cpp:227:13: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
  227 |             (void *)android_media_MediaMetrics_submit_bytebuffer},
      |             ^

See https://github.com/llvm/llvm-project/pull/150557.

Use a `const` variable instead of `constexpr`, matching the pattern in
other C++ source files in this directory. (There are many files that
declare a gMethods variable.)

Bug: none
Test: presubmit
Flag: EXEMPT bugfix
Change-Id: I5f0feb94d7fd531eafb3c9fc2df1cf05ac588f85