Move FtypAdjusterTest.java into muxer module

Changes include:
1. Move the test file into muxer module
2. Code improvement
2. Remove dependency from Mp4Slicer library

PiperOrigin-RevId: 511191076

testdata/src/test/assets/muxerdumps/ftyp_box_added_new_compatible_brand.dump

+ftyp (32 bytes):
+  Data = length 24, hash 87F07F95
+free (32 bytes):
+  Data = length 24, hash 84304D01

testdata/src/test/assets/muxerdumps/ftyp_box_changed_main_brand.dump

+ftyp (28 bytes):
+  Data = length 20, hash 44CF0210
+free (36 bytes):
+  Data = length 28, hash 294FE481