[le audio] Fix program info appended with unexpected character

NewStringUTF should be used with modified utf8 with null terminator.
In our jni layer, the code directly parsed program info from metadata vector
which doesn't end with null terminator. This caused unexpected behavior
of NewStringUTF.
The fix is to convert to string first before parsing.

Bug: 293637403
Tag: #bug
Test: atest BluetoothLeBroadcastTest
Test: manual test to confirm program_info not changed
Change-Id: Ic7d4eeb8cd0aea05e0f6a667e468d6ece509e657