Use std::mutex instead of android::Mutex

This CL includes:
- Stop using utils/Mutex and use std::mutex instead.
- Stop using utils/Singleton.

Test: minikin_tests passed
Change-Id: Ib3f75b83397a546472bb5f91e066e44506e78263

• include/minikin/FontCollection.h[diff]
• include/minikin/FontFamily.h[diff]
• libs/minikin/FontCollection.cpp[diff]
• libs/minikin/FontFamily.cpp[diff]
• libs/minikin/FontLanguageListCache.cpp[diff]
• libs/minikin/FontLanguageListCache.h[diff]
• libs/minikin/HbFontCache.cpp[diff]
• libs/minikin/Layout.cpp[diff]
• libs/minikin/MinikinFont.cpp[diff]
• libs/minikin/MinikinInternal.cpp[diff]
• libs/minikin/MinikinInternal.h[diff]
• tests/perftests/FontCollection.cpp[diff]
• tests/unittest/FontCollectionItemizeTest.cpp[diff]
• tests/unittest/FontFamilyTest.cpp[diff]
• tests/unittest/FontLanguageListCacheTest.cpp[diff]
• tests/unittest/HbFontCacheTest.cpp[diff]