Revert "Remove FontFamily.addFont and make FontFamily immutable."

This reverts commit 41e02e96131c1ec66d013e4615348be013518dc4.

Bug: 34378805
Change-Id: I8f1ee00b365c8b17c6140e9e286fbea082e31364

• include/minikin/FontFamily.h[diff]
• libs/minikin/FontCollection.cpp[diff]
• libs/minikin/FontFamily.cpp[diff]
• sample/example.cpp[diff]
• sample/example_skia.cpp[diff]
• tests/unittest/FontCollectionItemizeTest.cpp[diff]
• tests/unittest/FontCollectionTest.cpp[diff]
• tests/unittest/FontFamilyTest.cpp[diff]
• tests/util/FontTestUtils.cpp[diff]